。
The clasp won't catch properly.
Literal
Clasp [subject-が] well doesn't-hook.
留め金 (clasp/fastener) is a compound: 留め (fastening) + 金 (metal). うまく引っかからない (doesn't catch/hook well) uses うまく (well/successfully) with the negative of 引っかかる (to catch/hook onto). A small everyday frustration.