Was your light a studio light, are they that much more efficient than halogen floods?
I had to use three 500W floodlights like this
A twin stand from above left and a single (the same as the link) below right just to get the recommended light level, as recommended by FoCal. I would reinforce that you need the light level they recommend as a minimum in my experience, I was only just above their level.
Macguyver's post is perfect! I had problems with my 100-400L too. Some of it was due to insufficient or varying light. So I decided to do all my calibrations at night. I light up the target that is pasted to a piece of plywood with a 500watt light. I also learned quickly not to raise the central column in order to maintain stability.
If these two do not solve your problem, may be take it over to Canon... All the best.
Yup, my halogen light looked like that. Well, without the stand...
No money or use for studio lights. I mainly shoot birds and bugs and flowers and scenery (mostly sunrise/sunset). The halogen is from the time I had a party in the garden
I kept moving the halogen closer to the target until I got the right light level.