I bought this 14/12 second hand, so the problem may be specific to this unit or could be an issue for all owners.
I was changing the blade & noticed that the bottom wheel lock nut was only finger tight. When I tightened it the wheel actually moved back on the shaft by about a millimetre! The upper wheel nut was also lose, but not to the same extent. The blade now rides more central on the bottom wheel.
I also found that there was a point where the nuts could be over tightened. As a precaution, I put a dab of my best nail polish on the nut & the shaft.
Have any other owners found this issue? Peter