Why do I get shortened duration of Botox over time?

Well, 20 units is a low dose and there may be variations in how the Botox was injected. You might have to increase your does or try Dysport. See an expert! Good luck. Dr. Aldo

Since you mentioned you have active facial muscles, it is possible that you need a larger dose of Botox to obtain longer duration of effect.  20 units of Botox would be considered an average dose for frown lines and it would not be usual to use a larger dose for that area in some people.  It is also possible that the injection technique could affect your results.