CR-20 Pro - Z axis issue ? Thanks for your help

Hello all,It's my first time on this forum, so I'll introduce myself a bit.I'm a podiatrist using 3D printing in my work. Recently I acquired a CR-20 Pro on the advice of a friend.Truth is, I did had some problems with this printer (regarding the auto bed leveling and such) which are now under control. But there's something that bothers me. I printed some pieces and there are "lines" on all of these pieces at the same Z height. It's not much, I'd say 0.2 mm maybe and seems to occur on both axis.Has anyone ever had this issue ?