I have observed this in my game and noticed that when I click for her to go watch the awards, she waves and can't get in. I also noticed that certain "stars" have to enter first before it allows my sim to go in. My sim will be able to enter then (and they may be a notable newcomer or rising star) but it will let them enter after other stars have arrived and entered the building.
I now just take my sim there and let them decide when to go in. I don't keep trying.....I had to "let it go", LOL Seems the sims are smarter than me......they know when to wait and when to go :D
I noticed also that when the show ends, my sim goes to bar for a drink (usually the only one that sticks around). The bouncer remains and doesn't let anyone (fans) in unless they pay enough. Always fights at the rope! I have taken my sim outside and had her play the guitar or sing, and the bouncer then leaves, or if the bouncer stays the fans seem to go in the building with no problem. So I wonder if this happens because the last "star" came out?