Some lenders will allow you to pay a fee to re-lock your interest rate, other's won't. Generally speaking, once you lock the rate the mutual agreement you have entered into, assuming you signed a lock agreement is that if rates get better you will honor the rate you locked into. If rates get's worse, the lender will honor the rate you locked into.
The issue from the mortgage companies position is that when an interest rate is locked, it costs money if that rate is not delivered. The mortgage company actually hedges your rate in the marketplace, much like a builder will hedge on lumber prices to avoid rising costs during construction. Obviously the problem is that once you lock the rate it could go or down and no one knows for sure which. To do so would require a crystal ball.
In short, you are stuck with the rate you locked into and at least in my state, it would be illegal for another mortgage lender to even quote you interest rates when your are currently locked in with another company.
Look at it this way, if you locked your interest rate to let's say 6.5% today and tomorrow the interest rate goes up to 7%. Would'nt you be upset if the lender decided to give you the higher interest rate? Well they won't do that since you "locked" the rate.