I think you've got the ones with countersunk screw holes. I've not used those but I would guess they just need the screws tightening really well to create a little indentation in the wood, or try a little tap with a hammer with a block of wood between. Try it first on an offcut of 18mm brich ply if you have one, or do the bottom back corner first. Alternately you could get some corners that  don't have that countersinking, Fender branded ones don't I think.

Its also possible that the radiusing on the edges is not matched to the radius of the metal corner, that would prevent them fitting correctly. I think the standard is approx ½" radius.

A photo of the corner would help.