Skipping steps

Hi, everybody, first time here,
I´m new at RepRaps and im having a printing problem, the printer left little gaps between the layers in the final piece and i can´t figure out which may be the cause of the problem.
I think it mut be an spkipping step error in the z axis, but i can´t find any calibration error in the printer.
My printer has a WADE model extruder with a Magma nozzle and a hot plataform moving in Y axis.

I m atach photos of the final piece with the error marks.

i would be thankful if someone can help me!

sorry for my english!
bye

What material are you printing in? Have you noticed your extruder skipping at all?

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