Model not compatible with USDZ problem

Hello,

what may be the cause for a model not being compatible with USDZ? I have one model that can’t be downloaded back as USDZ and can’t figure out the cause.
Is there a list of not supported features that I can consult?

My model also says that its unavailable for USDZ despite it being available before. I don’t know what feature changed so I would also like to know if there is such a list.

Also, is there some sketchfab specific features that are not supported? For example, the lights, refraction opacity, ground shadow?

So far my models become USDZ compatible after a while, the internal conversion seams to take quite a while. Wait for 5-15 minutes after 3d properties changes.