Of the thousands of Botox patients that I have injected, I have never encountered a patient with immunity to the effects. There have been patients that require higher dosages, sometimes twice the usual amount.
The most popular Botulinium Toxin type A in use, and the one that you probably received from your 5 injectors is Botox manufactured by Allergan. You may consider using Dysport which will soon be marketed by Medicis as Reloxin. Mentor will be putting out another alternative called PurTox. There are two other forms Xeomin in Germany, and Prosigne in China.
If you have superficial lines in your glabella, a fractionated CO2 laser resurfacing is a great alternative, and a more permanent "fix". At age 53, you (like me) are showing other signs of aging like lax skin under the eyes, face and neck, hooding of the upper eyelids, crow's feet, and general sun damage of the skin. A single treatment with Fraxel re:pair, Active/Deep FX, or Mixto will give you overall skin rejuvenation, help you look years younger, rested, and healthy. If you choose to go with the fractionated CO2 laser, make sure to choose your treating physician most carefully. Be well.