This is a known problem on Lagunas apparently. When it fails completely and the bonnet won't open at all, it can be expensive to fix, so fix it while you still can. Some people have claimed it helps if you get a friend to selectively push down on various parts of the lid while you pull, perhaps trying to disengage the two catches one at a time.
It might be necessary to change the cable if it has become stretched, but simply cleaning out and lubricating the catches often seems to help. They work a bit like car door catches, and their operation should be obvious once all the crud has been wiped away.
Joe