Hi Graham,
Two things to check, firstly, the catch mechanisim may need to be bent slightly further back to engage better with the latch bar on the catch flap, this may take a little experimentation, so move it in very small increments.
Secondly, check the condition/existance of the black support peice, this should be mounted just inside the mouth of the catcher, over on the left side of the mouth, it's function is to help support the flap to prevent it pulling free of the lach during operation.
