I am sure you may be aware of this, but the TOSLink will only go in one way, it is tapered and has slide guides. Also I am unaware of your situation but trying to fit one of these things in from in front of the equipment is a bear. Look closely at the two little guides on the sides of the actual connecting part of the cable and ensure that there are no defects or dings in it that might have distorted the connector from it's proper shape. They want to fit in "Preceisly". They will not go in any other way.
I say this because it is possible that you got lucky with your other cable and just had it right when you put it in, not doubting your knowledge in any way.
Other than that I really do not know what to tell you, all of the connectors are the same, once properly positioned they should go right in. You could remove your Monster cable and compare the two closely. I have never heard of a defective end on this type of cable. But if everything else looks the same I would be curious to what the problem may be.
