ATARI 810 reads only first track

Hi there,

I have an ATARI 810 drive issue - it only reads the first track of a disk, if you try to boot:

I turn on my drive - the head moves correctly on the rails to it's starting position - I insert a DOS disk and turn on my Atari 800XL.
The drive starts and it reads the 18 sectors from the first track - I hear 18 beeps.
Then, the step motor on the right tries to move the head to the next track (track #2 ?) but fails.

Repeat
            {
                The head shakes a little bit, but remains on the original first track.
            }    
Until [Turn_Off_Drive]

I already cleaned the head, I cleaned and put some oil onto the rails.
When the drive is turned off, I can move the head on the rails and when I turn it on, the head is positioned correctly by the drive onto track #1.
I already cleaned all socketed ICs and all plugs.
Yesterday, all of a sudden it worked for 10 minutes, after I pulled off and reattached some connectors.

But now I have the same issue permanently.
Anyone has an idea?

Thomas