I solved the problem.
Bearing was defective. the nut that goes into the plastic part of the head had an axial movement of about 2 mm.
when the head is tightened by hand, everything is fine, but during operation, the plastic cap tightens by rotation, the shaft is pulled down and friction is created at the upper bronze plate. The shaft turns more and more difficult. I haven't been able to figure out why this is happening. maybe some manufacturing defect?
new bearing is €12. Not bad.
Thanks for your help