Does the button still pull up? Or does it seem jammed in the down position?
It's possible that the little link inside the main mechanism is either bent, or just gummed up with years worth of old junk.
If you can get a flashlight on it to get a better view, you might see something amiss. But really, it's not that big a job to just pull the whole thing out. You do have to memorize (or take pictures!) where every rod and clip goes of course, but once you have it out you can track down the offending piece. Or you might just find, half-way there, that one of your little clippy-doodles is busted.
If it's something un-fixable, then you can buy a new one. But if you can fix it yourself, so much the better. And you will be amazed at how good a properly working latch feels!
Good luck.
Paul