Also check that the lock-nut on the centre pin hasn't come loose (that greasy springy thing attached to the middle of the bonnet that you keep hitting your head on
)
This pin is threaded and can be screwed in or out to adjust how tight the bonnet is held shut at the front. I've had cases where the lock nut has come loose and the pin has screwed itself in enough to be hard to get it to latch into the lock on the slam panel.
Even if the nut isn't loose, maybe someone has adjusted it a bit too tight. Try loosening the lock-nut and screwing the pin out a turn or two and see if that helps it lock easier (don't forget to re-tighten the lock-nut when you're happy with the adjustment
)