Aiwa Z-M2600 HiFi - CD Player cant recognise all 5 CD's on the tray?

My father has a Aiwa Z-M2600. (Bought in 1998).

Although the CD player can play CDs, recently it has started to not recognise every CD out of the 5 which can be held at any one time on the "Direct Disk Play/Rotary Changer System".

Hence, it will ignore some discs while recognising others.

I intend to remove the outer casing and see if perhaps there is a sensor which is most probably coated in dust. If so I will clean it along with all the other internals I can access easily, if not I will have to come up with another plan.

Any other suggestions appreciated.