It's not a problem (for me). As soon as I "mouse-over" the blank image, the preview appears.
Same for me, I only (but often) see that during an import, when LR has done the file copy and sql referencing but not yet any preview rendering. Solved by waiting a few seconds.
It might well be that somethings hungs your machine in a frozen space-time vortex, or some similar oddity.