There are some reasons why Botox may not produce the expected results in some patients.
One interesting cause is that of developing resistance to Botox®: This is possible if you develop antibodies against the toxin. Higher doses (not in cosmetic treatments) and too frequent treatments can increase this risk. There are possible solutions to resistance. This can be discussed during the consultation with the practitioner.
Another cause is inappropriate use of Botox, for example using Botox to treat wrinkles that are NOT due to muscle contraction. Patients whose skin is deteriorated from sun damage have wrinkles that are due to skin quality being poor. They will also have pigmentation along the skin fold lines. These will not respond satisfactorily to Botox. For such skin aging, resurfacing and rejuvenation such as Chemical Peels, retinoids will be necessary.
A third cause that is becoming more common is overuse of Botox and other botulinum toxins causing Muscle atrophy or wasting. Just like any unused muscle weakens over time, weakening facial expression muscles with Botox repeatedly will result altered muscle dynamics over time.