I've been having problems with Steam not validating my games whenever i turn my computer back on, you may be having the same problems. I've been having to revalidate my CSS and Ep2 every time I've restarted my computer so textures and models will show up. Its been getting rather annoying if I'm honest, you may want to check this isn't your problem too.
To validate, just right click the game in your library, it should be in that menu, if not, click properties, and it'll be in there. Hope this helps.