Can't mirror in sculpt mode I saw tutorials on sculpting that showed the use of mirroring your brush on one of the axis. It's supposed to be as easy as selecting an axis and although it worked on a test it isn't working on the mesh I want.

The mesh I want to work on: https://www.dropbox.com/s/iprs2h62j0tf2m3/ghdsjf.png?dl=0

The test object I tried it on with the model I want to work on in the background: https://www.dropbox.com/s/e0ckohh8pun8thl/klhuyt.png?dl=0

I had the same default settings selected when I tried working on both meshes. 

Since it worked on the test it probably has something to do with the mesh which I didn't make myself - I downloaded it. 

It's really important that I get this to work because I need to add symmetrical details in the face but I don't want to use the mirror modifier because I already have made purposely uneven features in the nose.
Mate, your symmetry axis is set to Z it's the one from top to bottom (blue arrow) and it works on the test mesh, cause it's symmetrical on this axis. To sculpt on your final mesh you need to set Mirror to X axis (red arrow leading from left to right on your image)