Help with mirror modifier

Hey guys. I was modeling a laptop the other night, and as you know, a laptop is a symmetrical obect, therefore I need to use the mirror modifier. I applied it and then I applied a subsurf modifier to make the edges have a curvature. My problem is that, when I try to make a loopcut to soften my corners, in 1 direction the loopcut gets reflected the way I intended, but in another axis, it cuts only the half where the loopcut was made, not on the other side. I’m almost new to Blender, and I’ve created some succesful modelling before this one, but this is giving me problems, and I need to get this modifier right before I try more things at Blender. Do you know what my potential problem could be? And if so, can you help me with getting rid of such problem in the future? PS: In the first picture, the loopcut is reflected as intended, In the 2nd one, it’s failing and in the 3rd one, if I change my setings to mirror the X axis also, then the mirrored mesh clashes with the 1st mesh…

The mirror modifier uses the objects origin as the center point (though you can change it to another object, like an empty for example), object rotation is also important.

You need to cut your object in half and delete the side you want to mirror.
Here is an example: First one is the full object, second one is cut up to be mirrored on all axis, last one has a mirror modifier (using the second one)

I did cut my object, the problem is that one of the loopcuts I need doesn’t reflect on the other side
of the mesh