One of the things you could try would be to backup and/or rename your database and start with a blank to see if the problem is corrupt entries storing invalid waveform data. Make note of a couple of tracks you know have been scanned but haven't loaded their waveform.
Go to the following folder:
XP: C:\Documents and Settings\yourusername\Application Data\Pioneer\
Vista: C:\Users\yourusername\AppData\Roaming\Pioneer\
Win 7: C:\Users\yourusername\AppData\Roaming\Pioneer\
OSX: /Users/yourusername/Library/Pioneer/
And rename the rekordbox folder as rekordbox-working, then launch the program again and you'll see it come up blank. Import a few of those noted tracks and see if the waveform now appears. If so, the problem is the database. If not, the problem lies elsewhere, possibly with the file itself. Please let me know how you make out with these steps.
