Liquify stops working

Hi!

I'm not sure if it is me who is missing something, but seems like liquify effect many times suddenly quits working.

What happens is I don't get the "pen" that I draw with usually (when liquify works), but the cursor just stays like the normal cursor in AE. If I drag on the picture, nothing happens.

I thought I knew how to use liquify, is this a bug or could I be doing something stupid?