Yours will come off again after a few sets of stairs. Regardless, I keep using the challenger stair machine, because I haven't found anything better.
I reglue with gorilla glue every time it comes off. It also helps to cut about 1/4 of an inch off of the perimiter of the velcro circle. One of the forces taking the velcro off is force around the exposed edges.
Also, I now use 13 inch microfiber pads with the Challenger stair machine instead of cotton. They stick sufficiently to do the stairs with no problems, but they pull off the machine much easier then cotton pads.... the other source of stress that pulls the felcro off the machine.
One more thing. Score the back of the felcro the next time it comes off. That gives the glue some grooves to stick to. This plastic that the velcro is made of is impermeable to any glue known to man.
With these modifications, I now reglue the velcro after every 6 or 8 sets of stairs instead of after every 2 sets.
Challenger sells additional driver heads (the whole part that the velcro is attached to) for the stair machine and the standup carpet machine. You have to get in touch with the owner by phone to get them.
Another variation I've tried with my small stair tool is this. Throw the velcro away. Get out the Gorilla Glue. Smear it liberally over the pad driver of the tool. Then sprinkle some beach sand over the glue. Then, with a small plastic bag, or a piece of waxed paper, push the sand down into the glue. Lean the machine against a wall so the glue won't run off to one side before it fully dries. Now you have a nice rough surface that probably won't come off. Mine hasn't in over a year. Then use the Challenger stair tool pads, and one or two extra-wide rubber bands to hold the pad in place, and - voila! - a renewed stair tool. This is also my upholstery cleaning tool.
Note: I do like the Oreck Orbiter/Pro Model for large stairs.