Problem with mirror modifier and joining meshes

When I tried to add spikes to my Chupacabra’s back, I tried to join the spikes to the torso. But when I did that, it disabled the mirror! The abs and hunch were all still there, but only half the torso itself was there! I had to create a rough/crude second half of the torso from scratch in the interim, but decided to nix the spikes until a solution can be found.

Does the mirror not work on complex meshes? Thanks for your help.

Sound like you joined the mirrored back to the innkeepers spikes rather than joining the innkeepers spikes to the mirrored back. Remember the last selected object is the active one so the joined object will inherit it’s attributes.