@parveenkd
A checklist of things I try when they don't work:
Refresh site model by clicking 'Update'
Make sure site model is showing 'proposed' not 'existing'
Make sure site model is set to read 'all layers' in site model settings > general > use site modifiers on > 'All Layers'
Change texture bed to 3D poly, update model, then change back to texture bed. Sometimes resetting it seems to make it work
Make sure the class the texture bed is on or the render settings of your texture bed, has a texture. If there is no texture, just a fill colour or outline colour, it wont show as a texture bed in 3D.
Make sure all the layers and classes the texture beds and site model are one, are visible. Texture beds are only visible in 3D when the site model is on.
Make sure all of your texture bed is on the site model and not hanging off it. If it goes beyond the boundaries of the site model, it may not work.
This is all I can think of for now. Please correct me if anything I have said is incorrect. I hope this helps!