Mirror Modifier

Everytime I try to create the head of what is suppose to be a bear using the mirror modifer everything looks fine until I put on the sub-surface modifer and then there is this indent down the center of the face giving it a distinct “buttface” appearance. Simply moving the verticies across the axises does not work. I have redone this many times to no avail and was wondering if anyone had any thoughts on how to fix this.

without screenshot or blend file it’s a little bit guessing.

Is the subsurf modifier placed below the mirror modifier?
Do you have ‘Merge’ in the Mirror modifier checked?