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Zbrush 4R7 move problem

Hi everyone, i’m a Zbrush newbie and i’m using it for a university project. I’ve created a feather with alpha from a standard 3d cube, “extract” the feather with the morph target and doubled it with mirror and weld on the Z axis. My problem is that, as you can see in the video below, when I try to move that tool, the middle of it is “stuck” to his original position. What am I doing wrong? Thanks for your help.

Video:
https://youtu.be/j3pCYHDqung

tool>masking>clear