Did you drop the bonnet from a height to shut it?
chances are either the support bar bolt that attaches between the lock and crossmember has undone slightly (theres only one/two bolts attached at the lock end) and its letting the lock twist when you trying to lock it...or the whole lock mechanism as slid done very slightly...both just require undoing the bolts and repositioning the lock before tightening everything back up. (iirc the bolts are a 12mm.)
Or i guess you may just need to grease the lock. (try that first, cos its less fiddly
)