CEC CD3100 CD player tray belt, puzzled indeed!

Hope somebody has a good educated guess at what the issue is here because it has baffled me so far.

I bought a CEC CD3100 belt drive cd player as non working item.

Got it and as I suspected it was the belt for the tray door, in this case it was so stretched it was just slipping on the drive capstan. A quick down and dirty fix was to cut it and tie it back together in a knot which actually worked surprisingly well and allowed me to play cds to prove it was worth going further.

Could not find anywhere the exact size of said belt so measuring the one I had  ( before I cut and knotted) and deducting approx. 15% for stretch ( figure most sites seemed to state was normal stretch), I ordered a rubber belt from eBay.

Now when this arrived it actually fit perfectly and the tray flies in and out tout suite and I thought my job was done.

Ha, if life was only that simple!

After putting the top cover back on and fitting back into the system cued up the first cd and... nothing!

It spins the cd, reads the tracks and total time but then will not play at all.

Replacing the new belt with the old knotted one and it reads and plays cds again just fine!

My best guess so far is that maybe the new belt is a little too thick ( it was the thinnest cross section one I could find but it is a fair bit thicker than the original).

Now how or why that stops it playing the actual cd I have no idea.

Which is where I hope somebody much smarter than me comes in..... please.. lol.