Imported Model Sometimes Invisible

Hi all

I have an intermittent problem Unity. I have imported some animated, 3D human models (around 3k polys each) and bring them into my scene as prefabs. Sometimes (~20% of the time) they don't render. The game runs fine in every other way. I have a feature where you can click on the humans to view information, and if I click where they should be standing this still brings up the information. The frame-rate is also fine.

Has anyone else come across this problem? If I flip from my game viewport to my scene then back again the models all appear. The problem also exists in standalone builds as well.

Any help greatly appreciated!

Edit: I tried dropping the polys in my models to ~500 and the issue still persists...