Selector switch problem on new gearbox.

[TJX]

So I recently installed an APS gearbox in my DBoys M4 metal body. Everything seems to fit just fine, but after I fire semi once, the trigger gets stuck like it's on safe. I can switch to safe and then back to semi, but after the first semi shot, the trigger gets stuck again when I try to fire my second semi shot. I can switch to full auto and fire full auto just fine. But if I switch from full auto to semi, my semi mode fires full auto.