So do you think the problem is also that the targets are too flexible - thus absorbing the impact 

Filing the lip is interesting - never tried that !

Great advice Ian 

Yeah - apparently the targets can flex and basically bend backwards without the lip falling off the shelf.  I find it hard to believe, but I have noticed that the softer the targets the worse the problem tends to be.  The old Stern 'scrabble tile' design are definitely stiffer and drop heaps better.

cheers
Ian