PS3 not recognising discs?

Help. My PS3 does not sem to be picking up/recognising discs. The green light comes on when disc is pushed in. I can hear the drive spinning, but it is not reading. I thought that the laser may have gone, so I replaced the blu-ray drive with the drive from another PS3 (YLOD but drive definitely working). However this has not resolved the problem. I have the latest version of the software downloaded as well. I am struggling to find what the issue may be, please help!