Blend Modes not working

Hi everyone I am sure I have seen a post on this before but can't seem to find it! None of my blend modes seem to be working on my MacPro with Snow Leopard. I a running 10.6.2 and thought that might be the problem so I changed the quartz framework back to 10.6 from my MBP but still no joy. I have discovered I have the same problem with a Mac Mini running 10.6.2. So does anyone have any ideas, I need to use the Mac mini for a project starting tomorrow. Cheers Steve

MacPr Specs 2x 2.8 Quad Core Mac 8GB ram and an Nviada 8800GT. Quartz Composer 4 (103.1) Framework Version 4.0 (156.6)