Popular and modestly costed (under 150 Gems) parties often take as little as 5-10 SECONDS to fill and award Charms. By the time you see those parties on the Party board, you've probably already missed it.
This account has no Party Passes (spent them) and 346 Gems. Still, there are several parties in its price range, but they'll fill up fast. The modestly-costed Parties always do.
Vegas World strongly recommends using the "Remind Me" feature to more reliably get into any Vegas World party. If you click the Remind Me prompt fast enough (you have a few seconds) you will often get into the party.
Clicking "Remind Me" generates pop-up when the party starts, getting you right in.
REMINDER! If you wait more than 10 seconds, you'll probably miss the party. Click FAST!
Normally when you navigate to the room where the party is, you'll either get the "Bouncer" screen immediately, or once the party begins (if you showed up early, as some folks do.) If the room is already full the game will try to get you into a new party room (often referred to as overflow) but if that's not available you'll get a "Room is Full" message and in that case, you'll just have to choose a new party.
Every Vegas World party triggers when the room fills to 25 players. That means every party where more than 25 players try to attend (which is most of them) will result in an extra room with 0-24 players in it, and those rooms will only award Charms if they manage to fill. Sometimes they do, often they don't. If that room fills, a third room will likely then be created and those players may or may not get Charms. Since nobody is ever charged Gems or Passes until the Party Charms are awarded, all any of these issues cost is time.
It is possible for the meter to reach '25' while players are still entering the room. If any of those entering players don't get in, for whatever reason, their Gems aren't collected and the counter will drop back down to below 25.