Thanks, Kirk. I resaved the first .psd of a bunch that weren't rendering, and sure enough, it now renders in PM just fine. The weird thing is that the next 5 .psds in the group that hadn't been rendering now are, albeit in a very blocky, pixellated way. These 5 files appear to have been the ones on-screen when I resaved the first .psd. If I then restart PM, these 5 files plus a bunch more that didn't previously render are now rendering in pixellated form, although there are still some that don't render.
I understand the 'solution' is to resave all of these with 'Maximize Compatibility' turned on, but if PM is able to SOMETIMES show some kind of thumbnail (even if very pixellated) for non-compatibility maximized .psd files, why doesn't it always show it?
Thanks!