Ok this is what i do and it works every time for me:
All my copies are made with either Nero5 burned exactly as the tutorials say or for EA games using clone CD and the EA patcher, media is either memorex or sony and i burn at 8X which is as fast as my burner will go.
I have the Neokey in USB1 and the GS2 in the 2nd memory card just because a couple of games don't seem to recognize the memory card in slot 2 for some reason although it does work either way for booting.
To boot press the eject button (the PS2 should be at standby) insert the GS2 1.6 disc (make sure that the switch on the neo key is all the way to the left or down depending on which way you have your PS2 set up) then press the reset button. Wait until the PS2 logo appears then immediately after the screen goes black flick the switch on the Neo Key. You should now load the GS2 menu. From there, hit start game, then without codes, when it says to insert the disc press eject and insert your backup, wait for it to recognize the disc (for me the CD spins for a short time then stops) then press the X button and your off, it should load as normal - some games do take a while but they all load.
If this doesn't work for you i don't know what to suggest other than checking the soldering or your burner.
Fingers crossed
Daz