This can take ages - particularly difficult is to get the root of the reed correctly situated. The tip appears to be free, but it can't vibrate properly. Be prepared to replace a reed instead of spending hours messing about. If it works when removed from slot, but not when in slot, clear all paper out of slot, if that fails look for a reed frame that fits properly or carefully file slot out. First push reed tighter into slot if it is still loose cut a piece of paper to size and slide it into slot between the reed and the edge.
Gummed paper can be useful if the reed is to be removed again soon - gum it to the wood.
Be careful of overdoing it, this can cause the reed to jam in the frame. It will work when out of the slot, but after you've reassembled the instrument you'll discover it doesn't. Flap is Heard Before Reed Sounds Replace as necessary. Look for blackened or soft or curling valves, replace these first, again use a rubbery glue. See C. 2.