Product review (1 stars): Latch fails easily, but there is a fix
Had one of these a good two years before the latch failed, so I bought a replacement (and then, of course, the old latch started working again).  New one failed in less than a month.  From what I can tell, the spring is too weak to always bounce the teeth back up to the necessary position to grab the stem.  Spraying dry lubricant inside the latch did not help at first, but then it did start working a day or two later, after the lubricant dried.  The lubricant I used was Liquid Wrench Dry Lubricant spray, which leaves behind a dry white lubricant powder.  Evidently it reduces the friction enough that the spring can pop the teeth back into position after it's opened.

TEMPORARY FIX (until you can get the dry lubricant): if you manually pull the grabber teeth up as far as they will go, you can get it it to latch.  But then it will stick again the next time you open it, so you'll have to do that every time - quite annoying.