Two things
1. Check the socket fixings by looking up under from the spare wheel well. Mine once came loose with bolts falling out, and consequently, the hood would not release.
2. Have a look at the positioning of the hood pin centralising relative to the catch sockets. There is a slotted part on the hood frame, secured by allen head 6 mm bolts, that can be adjusted to move the hood back and forth. If they become loose, the hood can move and then the sprung pin does not centralise as it should.
