Please help with mask

I am trying to practice to make masks, this is the problem for me: I draw a simple red circle, convert to symbol, and put on the stage, in the mask layer I draw another square and this works well, but when I try to do the same with the pencil , when I scribble with the pencil to mask the red circle, when I lock the mask layer, it disappear completely,the same happens when I use the pen to scribble over the red circle, that is to say the circle and the mask.
Thank you for your help.

Vector Lines, or lines drawn by pencil or pen tool won't be affected in the masking. You'll have to use the brush tool or something else!