problems with digital dimmable ballasts for fluorescent tubes?

I've used digital ballasts before to dim hidden fluorescent tubes above kitchen worktops and it worked brilliantly. With coloured sleeves on the tubes you can create amazing effects at a reasonable cost. The latest installation has not been so successful. The tubes are flickering and sometimes won't start or switch off. I have to disconnect and re-connect them to start again. Tried new tubes - same story. Any ideas?