Regular examinations by the pediatrician should reveal whether or not there is reason to be concerned. I'm sure that your pediatrician checks the soft spot as a normal routine in the well baby check. Feel free to ask him about your concern during your next visit. It could just be the rate at which your baby's soft spot is closing. Some baby's growth progress in this area can be as late as 2 years. Some are earlier. Below is a cut and paste from a similar question asked and answered on parentivillage.com:
The anterior fontanel is the one most parents recognize as the soft spot. It's on top of the head toward the front. Four bones come together at this spot, but, because of their rounded corners, they don't quite meet. This soft spot may close as early as nine months of age or as late as two years. The average is between 12 to 14 months.



