Both Sides of Slider won't lock

Sounds like it might be a problem with the mortise. Before I recommend anything, you should test it out to be sure; remove the handles entirely and attempt to throw the mortise manually with a screwdriver. If the hooks are struggling to come out or don't come out at all, then it needs to be replaced.

If the mortise works fine then something else may be at play, but I'm not sure what it could be. Let's at least start with the mortise since that's the most common issue we see and work from there.

That's not something I can answer, unfortunately. You will have to examine the door frame and check.

If your current mortise is functioning properly then it could be an issue with the keeper that it hooks onto. Make sure that it is positioned properly and that it is engaging with the mortise as it should. If there are any other secondary locking mechanisms in the door, either at the top or bottom, you will also need to test those to ensure that they are functioning correctly.

The first step to fixing a problem like this is trial and error. You have to narrow down the list of possibilities until only one remains. Based on what you described, the mortise being the broken part seems the most likely culprit to me.