The drive key that connects the plunger to the drive gear can be off by 180 degrees. There are three flats and one curved edge. Assure this is correct. You have to disassemble to check, it doesn't need any special tools.
If the plunger is not moving, that is a sign the plunger is stuck at top dead center, this happens when the smallest bit of dirt gets in there. You have to be surgically clean when installing, microscopic dirt particles will jam the plunger.
Buttons come in different thickness, this determines timing and maximum injected quantity. Be careful here.