Do you have the long- or short-arm between the 396 and the suction cup?
If you have the long one, it may have too great a moment for the suction
cup to hold. I have the short one, and have only had it come off after
sitting in the sun on the ground for a couple of hours.

I haven't actually bought the suction cup yet, though I do have an
older one that I may try to adapt. I agree, the shorter, the better.
I think the one I have is maybe 6-8".