Donald Trump Jr. followed with a rousing speech for his father — but he soon yielded his spot to Ms. Brown, the actress who played the devious character Sheila Carter on “The Young and the Restless.” She was killed off the show in the 1990s, according to Variety.  “You don’t put an avocado farmer on in the middle of prime time,” said Russell Schriefer, who has been involved in planning Republican conventions for 20 years, about Ms. Brown. “That’s just common sense.”  “The most critical hour on prime time is 10 p.m. to 11 p.m. — especially at 10:30 when people change the channel,” said Tammy Haddad, a former MSNBC political director. “So you had Donald Trump Jr. hitting it out of the box with a good speech, but then it’s not just a soap star, but someone who was a soap star in the ’90s.”  Mr. Trump’s best opportunity will come Thursday, when he delivers his acceptance speech. And the pomp and emotional ceremony of that night could go a long way to erase any missteps of a convention that began with his wife, Melania Trump, delivering remarks that contained passages of a speech delivered in 2008 by Michelle Obama at her husband’s nominating convention.  “Seventy-five percent of the convention is Donald’s speech,” said Charlie Black, a longtime Republican strategist. “They have got to get that right. If they get that right, that’s the lasting impression that gives you momentum — or doesn’t.”  Still, the way the convention has unfolded to date has fed existing concerns among many Republicans about Mr. Trump’s political skills and the depth of his support in his own party.  The flat mood so far is no small matter, considering that one of the top functions of a convention is to energize delegates and party leaders before sending them home to sell the candidate. The meandering convention to date and the problems involving Ms. Trump are the latest evidence of the seat-of-the-pants organization that has marked this campaign.  On the third day of the Republican convention, the refusal by Senator Ted Cruz to endorse Donald J. Trump widened cracks in the party’s veneer.  In the best of times, conventions are celebrations, four days of speeches and receptions as a party gears up for a fall campaign.  The atmosphere has suffered from the absence of political celebrities — former presidents, presidential candidates, prominent senators — whom delegates would usually be grabbing for selfies and autographs. But Mr. Trump’s difficult relations with his party means that many of the party’s best-known leaders — former President George W. Bush; the 2012 nominee, Mitt Romney; and the 2008 nominee, Senator John McCain of Arizona — are not here.  Their absence has also limited the options of convention planners trying to fill four nights with speeches from well-known political figures who can pack the hall and keep television cameras trained on the podium.  Inside the hall, delegates milled around and kept speaking even as prominent Republicans took the lectern — among them Paul D. Ryan, the speaker of the House. While they cheered two of the candidates Mr. Trump had beaten to win the nomination, Mr. Christie and Ben Carson, a neurosurgeon, they greeted Mitch McConnell, the Senate majority leader, with scattered boos.  Ms. Haddad said she was particularly struck by empty seats. “People talk about the floor being a little low-key,” she said. “Well, the hall itself is not filled out. The best seats directly across from the stage are wide open. It’s significant. When a hall is not filled, it’s hard to bring the energy up.”  Protesters, reporters, delegates and security personnel converge this week in Cleveland for the Republican National Convention.  The lack of enthusiasm could be seen on and off the convention floor. Some hotels reported that guests had checked out early. Bars that had hoped to be serving revelers until 4 a.m. — thanks to a new state law passed specifically for the convention — have been closing early. A Rick Springfield concert the other night drew a sparse crowd.  Christian Berle, a delegate from the District of Columbia who supported Gov. John Kasich of Ohio, said he has found the convention so distasteful that he was prepared to pay the airline change fee so that he could leave early. “I mean, $400 to get out of this place seems like a cheap thing,” he said.  Norm Coleman, a former senator from Minnesota, said he would head home Thursday morning. “I have a lot of family commitments,” Mr. Coleman said with a smile as he stood in a convention hall marked by patches of open space.  A political convention cannot be judged only by action in the hall. Cleveland was teeming with lobbyists, many of whom chose to spend their evenings at restaurants and receptions with elected party officials rather than on the convention floor.  Andrew Richner, a delegate from Michigan, said the lack of energy reflected the fact that Mr. Trump had won after overcoming a crowded field.  “We’re still feeling the lingering effects of a 17-way primary process,” said Mr. Richner, who had supported Mr. Kasich, who has stayed away from the convention. “It takes some time to get over that.”  Pete Hoekstra, a former congressman from Michigan, said the lack of enthusiasm could be traced to the fact that many of the people in the hall were Republican Party regulars who were never going to embrace Mr. Trump.  “It clearly hasn’t been his crowd,” Mr. Hoekstra said.  A version of this article appears in print on July 21, 2016, on page A12 of the New York edition with the headline: Missing Many Party Celebrities, Sessions Lose Energy and Delegates. Order Reprints| Today's Paper|Subscribe
The Republican National Convention has struggled not only to make the most of prime-time speaking slots, but also to retain delegate attendance.