Try to get someone to press down on the bonnet, above the catch whilst someone pulls the release handle. Do not use a constant force pushing down, vary it, and you should hear it click release. Lack of lubrication in the latch is the likely culprit. Failing that, it may need a slight adjustment. The latch is adjustable for height, as are the rubber stops at either side of the bonnet. They all must be about right, otherwise there is too much strain on the latch whilst trying to pull it open.