Tray slow after mod fitted

This is really wied ? I have chipped loads of PS2 with the X key 2.5 DVD mod and I have come accross 2 that suffer tray problems. They work fine using the eject button but under the swap they are slow and don't always fully eject or return. it's not the mod chip or the wiring, if you swap the cd unit for a known good one it works fine.

Any ideas out there ?