I have a 1998 Ducato, but not sure if it is the same mechanism as the 2002, but a while ago I had this same problem.
I can't remember how I managed to eventually open mine, but it wasn't from underneath, so must have been teamwork in pulling lever whilst someone wriggled bonnet.
Until I sorted fault, which took a while, I attached a bit of wire to the release lever and tucked it between headlight and bonnet as a emergency release.
I tried everything to sort problem but I now *think* it was due to a misalignment between the hoop on the bonnet and the jaws of the catch, so that the bonnet hoop jammed against one of the jaws and wouldn't spring up when the release cable was pulled.
I said think because I'm not certain this was the problem, just that after many days of adjusting catch & springs plus fitting a new cable, all to no avail, bending the hoop to centralise it in the jaws seems to have finally sorted the problem.