Suction cups and reefs tanks don't mix well. Try and pick a spot with low flow. Make sure the glass is really clean, no algae. Make sure there are no air bubbles trapped in the cup.
I was having trouble getting a maxi jet to stay in place in my fuge a few weeks ago. It tooks 5 or 6 times of trying different setups with the pump. Now its no problem once I found the sweet spot. Hasn't moved in weeks.