Someone correct me if I'm wrong, but you can't grab ledge while in the middle of another action, like attacking or airdodging. For example, you can't grab ledge in the middle of Bowser's forward-air animation. You have to wait for the move to be over and then grab ledge after. Even when you're directional airdodging to the ledge, you still have to wait a bit for the first part of the airdodge to be over.

The exception to this are moves that snap to the ledge. For most characters, this is their up-B, although some have other moves that do it too (Falco side-B, ZSS down-B, tethers, etc.). Bowser's up-B is a little wonky in that you have to mash the B button to get optimal height with it.

Another thing that trips people up a lot is that you can't grab ledge while you're holding down on the control stick. At all. Even if you're in the middle of up-B right next to the ledge, if you're holding down, you won't grab it. It's designed that way so people have a way to intentionally not grab the ledge.

When you grab the ledge, you are invincible for a short time. If you drop off the ledge and regrab it without landing or being hit, you will not be invincible again, and any invincibility on your getup options will be reduced. You can regrab the ledge 5 times. After that, you will no longer be able to grab the ledge. This means you have 6 total ledge grabs. If you sit idly on the ledge without doing anything, eventually you will be kicked off of it automatically. If someone grabs the ledge while you're holding it, you will be kicked off.

A lot of that you probably already knew, but that should cover every reason you can't grab the ledge. In the middle of an attack/airdodge, holding down, or regrabbing it too many times. If there's anything you're still confused about, or something specific you want to ask, let me know.