Seho Selective Power System - problem with Z axis

Hello, I would like to ask you for help. We have problem with our Selective Solder System SEHO PowerSelective. Problem is, that Z axis is decreasing. When we start soldering, height of Z axis is correct but after each solder cycle, there is decreasing of Z axix cca 0,1mm. After cca 5 cycles, there is contact of pins on pcb with solder nozzle. Mechanically it seems there is no problem, belt if fixed, and there is no loose in fixing system. It seems that encoder or servo system is not correct. I would like to ask you what can cause this issue? And are we able to test our servo system if they are OK? We have tried swapped servo control unit with another axis, but problem is still. Thank you for your support.

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