First, is this a leaf switch or microswitch?
If leaf switch:
a) make sure the blades underneath aren't sticky
b) make sure there isn't a wire underneath that the button's c-clip is catching one
You can pull the c-clip off the bottom and then pull the center out of the housing to look for anything that would cause it to catch.
If microswitch:
a) make sure the switch is clicked into it's holders properly
b) make sure there isn't a wire underneath that the button is catching on
c) since it is longer than the original leafs, make sure the button isn't pressed up against anything underneath
There are two clips that poke out the bottom sides (one actuates the switch). Squeeze them in and push up, and the center will pop out so you can look at the interior to see if there is anything that would cause it to stay down...