toggle hold weights to on. .

or if u are doing smooth skinning, use the "bind selected joint" method, and choose only joints the may accfect your skin.
[ie. for fingers, all finger joints and wrist joint or possibly a forearm joint- may affect your hand skin. u can then mirror the weights across on the other hand.]

for the mirror to work properly, you may need to bind both hands at the same time to their corresponding joints.
this method is a little tricky, because of how maya displays selected joints.
otherwise use the hold weights options to make life easier.