Biasing an Ashdown CTM300

[la bam]

Hi all, can anyone help with a biasing issue on a CTM300?
I'm trying to bias a few valves, but they wont go to "0" Their limit seems to be 5 and 3.
They will go further down, but not further up?
I'm turning the adjuster anti clockwise to try and get the needle to 0, and turning clockwise brings it down towards -10.
Tube 1 = 3.
Tube 2 = 0.
Tube 3 = 4.
Tube 4 = 5.
Tube 5 = 5.
Tube 6 = 0.

Ideally they should all be at "0", but I just cant get that to happen. Am I doing something wrong?

If @Ashdown Engineering could help thatd be great too.

[la bam]

Sorted.