The pickup can be adjusted through a pretty good range on most setups
(ATB bikes can have a pretty large gap also). Another alternative is to
look up some 1/4 or 3/8" neodymium disc magnets and stack them on the
spoke magnet to move the magnet closer to the detector on the fork.
If the initial magnet is not 'strong enough' a dab of goo or instant glue
should suffice. Neodymiums have enough strength to hold themselves
in place.