Sometimes after childbirth and pregnancy your cervix will move or change position.
My girlfriend can no longer use tampons at all for this reason.

Make sure you are putting it in far enough to begin with.
What youa re describing is the feeling of it not being in far enough, but if i is, then your cervix may have tilted or moved. It may never go back to how it was before.

Also try getting some mini and some slim tampons and see if either of those work for you - it may just be a case of using different size tampons now. (Steer clear of tampax lol)

HTH