This is a recurring problem but sometimes it happens, sometimes it doesn't. When previweing clips in Icon view on any BIN, Premiere might decide that it doesn't want to show the thumbnail for certain clips and shows a grey square instead. As I said, sometimes, it takes a bit of time until they become proper thumbnails but sometimes they stay grey forever. All clips have finished loading and conforming so that's not the problem.
Is there something wrong with my preferences? I have quite a powerful workstation so i guess PC power is not the issue
thanks in advance
I can't think of any reason this might happen, outside of the clip not yet being loaded.
Just load it into the Source Monitor for preview. (Or use Bridge, if it works with the clips.)