Are There Different Strengths or Concentrations of Botox?

Each time you get Botox, providing you get reinjected just as the muscle function begins to return, you should get the same effect or even better over time. If the effects are not lasting as long it is likely the the number of units injected has been changed. You need to verify the number of units are are receiving and thus paying for.
I'm not sure why the increased pain. Although the 30 gauge needle that is used gets dull very quickly and this increases the pain with each successive puncture. I often have to change the needle once or twice with each patient just for this reason.

I totally agree with the other doctors. In addition, as noted, we usually dilute Botox to make it ready for injection. We can use different solutions for dilution, for example saline with or without preservatives. It may affect the pH (acidity) of the Botox solution, and the more acidic it is, the more it will sting. If the solution stays without use for a long time, it may also lose effect, at least partially. It may happen if the office uses Botox only occasionally, and keeps the bottle open for a long time.
I would go back to the doctor's office and talk to them about these two possibilities, in addition to the number of units used. The feedback is always useful. Hopefully, it will work better for the next injection.