Sims keep leaving party around the same time every night

I'm not sure what the problem was. I tried everything I could think of. There were no locked doors, the pool was accessible and almost right when they arrived I had them eat first. I also had plenty of stereos throughout the house playing music and 4 kegs. The only Sims that stayed were the 3 that were playing a game of pool with my Sim. Oh well :/ Those Sims stayed for a while at least. Thanks Igazor.