I was inking a piece and wanted to use the mirror tool. However,when i turned on the vertical one and did a stroke,the stoke did not appear on the other side. I tried again with the horizontal one but it still did not work. I even went as far as to create a new canvas and still the stroke won't appear.
I believe you are experiencing a bug, it can be your version it is around 4.1.1 nowadays, if possible talk with developers in https://krita.org/en/irc/. You can use the multibrush tool (Q) if it is working for you. It have a lot of extra features and some disadvantages (traditional mirror work with all the tools as it is not a tool itself). You just need to adjust it from simetry to mirror