I've experienced the same with the suction cup on painted aluminum in my RV4 just below the instrument panel and forward of the control stick. Guess the surface is too rough to hold for long as it rarely makes it to one hour. I've tried on the canopy with the same result and on a different, painted surface inside the cockpit.
Instead, I used industrial velcro and a small clip board for a purpose-made iFly kneeboard. Only problem now is readability during the middle of a sunny day in Texas, especially when wearing a light colored shirt. Reflection makes it hard or impossible to discern detail on the screen.
Otherwise I love the unit.