Certain props not showing up in VHE 4.0

[Coozins]

Some prop models are not showing up in-game in a map I'm currently making. For some reason, the ones not showing up are mainly Counter Strike: Source models, but some from Half-Life 2 and its episodes aren't showing up, either. (I've set my game configuration up so that content from those games can be used, so that's not the problem.) There's nothing that I can see that is different from the models that do show up in-game. If anyone could suggest any ways to fix this problem, I'd be grateful.