Usually keys catch because of wear at the pivot. Depending on the nature of the beast, this may be easy or difficult to fix. If it says Made in China, or is of reputable make, this is probably best left to a repairer. If it is of primitive construction it may be possible to sort it out simply, and may only need the pivot packing out with felt. Filing between the keys is likely to be of little use, as the worn/bent pivot will allow the key to swivel - any advantage will soon be lost. If damp is the cause, keeping the instrument in a dry atmosphere for a few days should restore its function.