2.9. Joined my Armature and now my Weight Paints don't work PLEASE READ THIS FIRST
I know you lose weight paints when joining armatures. This is NOT what I'm talking about. Okay, on to the issue:
PLEASE READ THE ABOVE FIRST
Hello!
I am able to color the vertices just fine, but when I go into pose mode the mesh doesn't move at all.
Here's a step by step of what I did:
My separate armature pieces had 0 weights. I had tested them prior to joining them and the weights worked just great. I deleted all the weights, applied all transformations, and then joined all my armature pieces together in one Ctrl+J key press. All seemed good.
Now, when I weight paint things get weird. For one, if I click the armature and shift-click the mesh and go into weight paint mode I am not able to select bones as usual. Secondly, and most important... the weights don't do anything! I am able to color the vertices just fine, but when I go into pose mode the mesh doesn't move at all.

Is it a naming issue? Parenting? I'm new to Blender so I'm likely overlooking something very important.
Thank you!
Weight paint only allows you to select bones on an armature that is linked to the mesh with an armature modifier, so you need to make sure that that is set up properly before weight painting.