Strange. I have had a set for a couple of years and love them. Have never once had them come off, and I leave them on for up to a couple of weeks straight without removing or having to re-apply a vacuum. Out of habit, I do check them each time I hit the road.
I think that you have to pay attention before setting the suction cups up - I keep a dishtowel in the truck and use it to make sure that the area where the cups are going is clean and dry. I also keep the cups themselves clean. When properly maintained and attached, I absolutely cannot pull them off.
In fact, they stick too well sometimes. A couple of seasons back, at the tail end of a trip when I was dead tired, I had a couple of Meisers up on the racks. Coming up from the river on a narrow track road, we met an oncoming vehicle. Without thinking, I stopped and reversed off-track to make way. A large pine branch slipped up under the two rods on the roof cup set. In seconds (but it seemed like slow motion), looking out the windshield I noticed the rods flexing down near the handles. By the time I realized what was happening and jammed on the brakes, bot rods had exploded from the flexing stress in the third section down from the top. The Sumo suction cups on the hood and the roof hadn't budged at all. If they had, the rods would have survived.