Models Not Appearing In-Game

This can occur in two different circumstances:

First, make sure you haven't accidentally altered the fade distances of the model so that it is immediately fading out without rendering. Open the Object Properties dialog for the prop entity, and look at the Start Fade Dist and End Fade Dist keyvalues. These values control the distance at which a prop will fade out, to help control performance. To set a prop to never fade out, set the Start Fade Dist to "-1" and the End Fade Dist to "0". These values can also be set by the concentric circle helpers surrounding the prop entity in the 2D windows.

Another possibility is that the wrong prop type is chosen for the model you're using. If you use a prop_static entity, and then specify a model that has physics properties (which should be a prop_physics), the engine will delete the model. To fix this problem, either switch the entity to prop_physics, or change the model to one that does not have physics properties. See Overview of Prop Types for more information on the different types of prop entities.