Hard implants are a dilemma faced by many women. There are a few possibilities. Capsular contracture, occurring in up to 10 % of women with implants, can make the breast quite firm. If this is the case, vigorous massage and the medicine Singulair or Accolate can help. If you can feel your implant and move it around beneath your breast and muscle, this is less likely, but a physical exam will help. Some physicians overfill saline implants to reduce the incidence of rippling that saline implants cause. This bears a trade off. The more saline implants are filled, the more firm they feel behind the breast. It would be a good idea to return to your doctor and have him/her take a look.
Good luck!