Moving app to system app failure

[Bender42]

Have a nexus 7 2012

Rooted with Nexus root toolkit
Checked with Root checker

Having issues moving an app from app to system app

When trying with ES file manager I get the following "Move Task Failed" no other explanation.
When using /system/app/mover i get Error: Segmentation fault 139

This is my first root and I am not the most tech savy person so will apologize in advance.

Can any help me with this problem?

Thanks