Textures unload unexpectedly

Hi, I’m facing some weird problem. I’m using plist spritesheets and it’s all ok. But sometimes it feels like some textures are removing from cache or something, because the sprites look transparent, no textures loaded.
Sometimes sprites appear with Default/ImageFile.png instead.
I don’t do anything to release cached images.
Can someone give me the clue or give some direction for further investigation?

Guys? Somebody?
@pandamicro