Major Waveform building problems

Hi, I've got PT11 running on an iMac (10.8.5 / 2.7 i5 processor / 20GB RAM)

and it's just started being completely unable to build waveforms of audio that I drop in from Finder....



I have to drop things in, save and close the project, reopen it in PT10 which builds the waveforms and then save and reopen in PT11. It's time consuming!



Anyone had this or know how to fix it?