Brushes disappearing or not working.

Hi

I am new to infinite painter and have ben following Ellie Taylors excellent video tutorials. I started using the Svetlana brush under the paint brushes and al was working fine until it suddenly disappeared. If I go into the paint brush category and scroll down I can see a box labelled Svetlana but there is no image of the brush stroke and if I click on it and try using it, nothing happens.  checked that I hadn't accidentally turned the opacity right down or made it so small that I couldn't see it, or was using a pale colour or a locked layer but none of these were the problem and if I click on another brush it works!

Then more recently I was doing another art piece and was using the Pollock Brush and ten it stopped working, This time if I go into the category I can see the brush and the stroke but it doesn't work. Checked everything again and cannot find the reason. All other brushes are working fine.

Am  inadvertently hitting some command? I have tried shutting down the programme and my android tablet with no effect. I have not tried re-installing the programme as I do not want to lose my work and it's layers. Any advice would be greatly appreciated.