Artisul Tablet and Sculpt Mode

Hello,

I am trying to use my Artisul D13 tablet in sculpt mode and I am experiencing a weird problem. When using the “Add” brush, it sometimes works but sometimes is replaced by a red dotted circle and doesn’t work. From what I found, this red dotted circle is the grease pencil eraser?

The subtractive brush always works properly. And what I found is that after some time of idling, the additive brush works again but if I press Ctrl+Z it stops working.

Does anyone know why this is happening and what I can do to fix it? I could revert to using only the mouse but I really want the touch sensitivity of the tablet.

Thanks.

Maya