VST not loading correctly

Anyone else had this issue? I load a VST and all I get is a blank grey box. I can still load presets from the bar at the top, but the actual body of the VST window is just a lovely shade of grey!
 
This is happening on Stillwell VST's and the ones packages with Sonar (Sonitus).
 
The project I have open is not massive (well I don't think anyway, maybe it is). There's 7 drums audio files and one drum MIDI, 2 guitar clips (spit over 6 tracks (intro/verse/chorus)), 2 piano MIDI tracks which feed into 2 audio tracks, a bass guitar clip (split over 3 tracks, same as guitar) and a vocal track. 
 
In terms of existing plugins, all tracks has the T-Racks EQ applied, although I turned all these off and still struggled. When I open a new blank project the VST's work fine, just this project I am struggling with.