How weird did the button feel when you depressed it the first time?
If it has any spring resistance at all, I'd attempt to center it in its hole using my thumb(s), since it looks like it's sitting slightly to the left in the pic you took.
(Perhaps it's just stuck under/against that left edge). This would be the forward (muzzle) edge relative to the receiver itself.
Failing that, I'd spray penetrating oil around the sides of the button, hold the receiver assembly upside-down, and tap the back of it with a non-marring hammer (or tap the receiver against a large stationary piece of wood) in an attempt to dislodge the button.
You can also go in there with a tool and push it up, but depending on how/why it's stuck, that may scratch or bend something.