I've had trouble since upgrading from LR3 to LR4 with previewing files on import. Sometimes reports no import, sometimes just shows thumbnail; sometimes fails on all photos, sometimes on just some. Happens for different file formats (ie, JPEG, raw from different cameras), different card readers, different USB ports. Canceling the import and selecting import again seems to be a workaround, but this is annoying nonetheless. There have been multiple complaints about this problem but no definitive answers. I assume it's a bug. Adobe -- anyone there?
Is the import dialog open when you plug in the card reader or camera? I'm tracking a possible problem with it recognising duplicates when the import dialog's already open, and this could be related.
No. I plug in the card, and then click import to open the import window. Have also tried plugging in card before LR is open, and while open -- no difference. I've noticed that LR4 is also temperamental about noticing the drive; in LR3, it reliably opened import automatically. BTW, one thought I had was that this might be a conflict with some built in Apple plugin that's contending on the card reader, but I've checked that (AFAIK) no action is setup to start when a photo card is detected.