Convert Lead button on Lead form ribbon is no longer visible

Hi all, 

I asked the opposite to this before, but when I made it thanks to this community the button gone forever (I guess, but for then I coulnd't see this button anymore.

First, I went to Settings> Customization: Solutions> Lead> Web resources and created a new one with a JS function to compare the existing security roles IDs with the logged in. Then, I used XrmToolBox> Ribbon Workbench 2016 to create an Enable Rule with the previous JS function.

It was working for a while, till we figured out that the button was no longer visible for one user, not for all.

The button is ConvertLeadQuick.Web.

Of course, I deleted and published any changes made on Ribbon Workbench, and on Customization Settings.

Any suggestions on where should I look at to understand where the button is hidden for one user? (The security roles for the entity are the same both).

Thanks on advance.