You said "just in preview mode" -- is this a preview issue, or do your transitions render incorrectly as well?
There have been several problems in the past related to HFX -- one was, if you were using OpenGL rendering in the HFX editor, some transitions would be distorted (too tall/cut off). The workaround was to NOT use OpenGL rendering in the HFX editor.
It seems that there may be some problems with some "purchased" HFX effects, too -- problems with the background images being missing or "distorted" on some machines.
I think that in order to determine what's going on, there needs to be more information shared. WHAT effects are having problems? Do these problems occur only in preview, or do they occur in rendering as well? Can other users reproduce the issue on their machine, etc.
Otherwise, it's going to be a bit difficult to get to any sort of resolution, as it isn't exactly clear as to what precisely is going wrong.
BittMann