It could be the two release springs above the grille (one on either side). I don't know if they are adjustable but it's worth having a look to see if one is perhaps loose or something which would cause the bonnet to sit slightly higher on one side.
The other option is the slam-pannel itself isn't sitting level on the chasis. If I recall, the slam-pannel and bumper can be moved slightly on the chasis mount to allow you to adjust the gap that is present when the bonnet is shut. It may be that the slam-pannel and bumper assembly aren't horizontal.
Another option could be a buckled bonnet, which is rather more difficult (and costly) to fix (ie. new bonnet
)
Hope this helps!