NAD 521i Troubleshooting

Mine has the same problem. The reason seems to be simple:
It is a belt that drives the movement of the cd tray and the elevation of the cd drive to pick up the cd and spin it. The belt is not producing enough friction ...... And that is why the cd deice does not the correct position preventing it from spinning the CD. Easy fix: replace the belt. You have to open the player and get the tray as far out as possible to see the belt. Just turn it around and it will work again.

.......another solution if the rubberbelt needs to be replaced: get any old CDROM drive out of a junkyard computer, open it up and look for the motor and the belt that drives the cd tray - there is usually only one. Get it out and transplant it to the 521i. The steps are: get the cd tray into open position, unplug the player, open the housing (5 screws), push two small lids close to front panel left and right to the cd tray and you can pull the tray out a little bit further, use a screwdriver to press down the cover over the belt-drive, lift it a little bit, remove the old belt and put the new one in, push the cover back into place, push the cd tray back into position and close the housing again.......done. Works great with mine and is done in less than 30 min!

This may help

reconditioning the belt
http://www.diyaudio.com/forums/digital-source/123669-quick-slipping-belt-fix-cd-player-mechs.html

I think the replacement belt is
http://www.mcmelectronics.com/product/42-1805&CAWELAID=220418241
sony replacement belt
SONY 3-653-387-00 BELT ( 365338700 ) OEM ORIGINAL PART