Apologies if this seems too obvious - did the seller include the little weight on the fishing line? Not sure if you were referring to adjusting that or the big counterweight on the end of the arm. I've had two tables with the pro-ject arms and they are unusable without the fishing line weight. Only other guess is the arm is not mounted quite right, perhaps the horizontal bearings may not be level even though the table is . . .
Jon
Jon