Having a problem in blender, seems related to mirroring

I mirror a mesh a lot, making a change to one side, liking it, halving and mirroring the mesh over and over again.

Anyway, I seem to get a problem when the mirror mod is NOT on, sometimes I have issues where, for example, I duplicate a vertex group to make a rim around an opening, and then I duplicate the vert loop and pull it out half come out fine but the other half is attached to the original mesh and other issues, like when I extrude some times half the mesh extrudes, the other doesn’t.

Have I got a setting messed up somewhere? Anyone else have this?

I’ll try to help if I can. Try turning off the clipping function in the modifier panel.