Are you certain the plunger is 100% fully out, mine is always getting stuck in, but this is due to roofing tile grit covering the boot and lock, and jamming it, I have to sometimes use a block of wood to thump the button in to get boot undone.
They are very easy to take out, only 5 minute job, not even that fiddly, but watch out if it's windy, as sapphy boot smacking you on the head hurt!!
I'm changing to a 4wd flush boot lock owing to where I need to park my car.
tabetha