So this is happening...any reason?
Trying to find ways to fix it, but it keeps going...
Are you sure the verts on that line are exactly on 0? It looks like they're just slightly off, so if they're displaced more than the Merge Limit of the mirror modifier, they won't merge. An easy way to fix this is to have clipping turned on in the mirror modifier and move the verts to 0 on the appropriate axis.
The mirror modifier has a clipping option that you can turn on. It forces vertices at the "imaginary plane" where the mirror occurs to not cross that plane. An easy fix when this is happening is to select all vertices that should be at this plane and move then towards the plane (so if you mirror in the X direction then move the vertices towards the mirror along the x axis).
You also need to ensure you have no faces made at the mirror plane. Hope that helps.