Also check the sides of the forend lug on the bbl and the corresponding inside edges of the cut in the forend iron that it drops thru for interference.
The iron may be ever so slightly bent and rubbing hard on one side or the other making for the very tight fit.
Or the wood inlet for the bbl shape on one side may be a bit high at the muzzle end of the forend. This will push the entire F/E assembly over to one side as it seats causing the same problem as above.
Needing to seat it 'firmly' to get the lock to engage and button to extend from the front can be then simply wedging the iron into place betw one side of the lug and that cut out.
Especially if you see the lock button doing a slow walk toward the front position instead of snapping into place. It's being bent very slightly also and not as free to move as it should be.
The lock surface on the forend latch is a beveled plane not unlike the bolt locking surface. It locks with a 'snap'. The bevel to take up wear.
If it won't go into position at all untill you start pushing hard on the somewhat flexible steel and wood component part,,then you are bending something there to make it position itself.
If, as DerAmi pointed out, the F/E iron has been stretched, (we don't know that it has of course), the punch and judy stretch methods that nicely elongate the narrow front section of the iron can also make it take a gentle bend or twist.
Though the mis-allaignments may be hardly noticable, they're enough to cause a once nicely fitted iron that dropped over the forend lug to now drop over only if it's forced down upon it. The latch has to follow that bent or twisted iron as well and doesn't as smoothly as it should.
Once on and latched, it usually takes a bit of prying to separate again.
Getting the iron assembly out of the wood and trying it on the gun will tell you a lot.
Just my thoughts...
