Disappearing sim models

[J-Heli]

I have disappearing sim models! I'll use them, switch to a different model, and then quit the sim for a while. I'll come back, say, thrity minutes later, and I try to select that model, but it says it's not available, so I go into the midel parameters, and try to find the appearance of the model, and it's not there? The actual model is still in the folder where I saved it to when I installed the sim, it's just the model appearance that's missing. This has happened to a couple of planes and heli. I've got no idea what the problem is? Thanks again.