Diagnosis
The magnet isn't strong enough for the surface, or more likely, the surface isn't actually ferromagnetic (steel or iron). Many exterior wall mounts, siding, and brackets are aluminum or vinyl, which magnets won't stick to.
How to Fix It
The mount includes a strong adhesive metal disc for a reason. Clean the target surface thoroughly with rubbing alcohol, let it dry, peel the backing off the adhesive disc, and stick it firmly to the wall, siding, or soffit. Press for 30 seconds. Wait an hour for the adhesive to fully bond. Then, the magnetic ball mount will stick powerfully to that metal disc.
If That Didn't Work
If you're trying to mount to a painted metal gutter or downspout, the paint might be too thick. Lightly sanding a small spot on the back of the metal disc (the side that touches the surface) can sometimes improve the magnetic connection, but using the adhesive is the intended method.
Heads Up
Don't rely on the magnet alone on a curved or vertical steel surface unless it's perfectly clean and flat. The camera and mount have some weight, and wind can easily knock it off. The adhesive disc is the primary mounting solution for most non-magnetic surfaces.