hi K I 7V G Z
What battery do you have in the front grip? The make I mean?
Only reason I ask is because we have found with some cheaper batteries, they are slightly bigger in size to normal duracel types, and sometimes this can push on the button, causing it to not depress.
Take the battery out while still connected and see if the button works.
If its not this, check the button (while the battery is out) to see if its got stuck somehow. A small piece of paintball shell wedged in there could stop it working.
Let me know how you get on please