catch onto (something)
1. To snag on a particular surface, item, or material, usually with frustrating or unpleasant results. Be careful so that the zipper doesn't catch onto my dress—it's silk!
2. To latch onto something, usually as is intended. You probably need to get your necklace repaired so that this little piece catches onto the clasp securely.
catch onto something
[for something] to snag on or get grabbed by something else. (See also something">onto something.) This little piece has to catch onto this gear each time it comes around. A few threads of my shirt caught onto the cactus needles and ruined the whole thing.