I'm glad you asked this because there are several different reasons.  I'm assuming you went to your usual doctor and that they are mixing the Botox the same way each time.  If the dilution were different, that could explain it.

 If this was a new area for you, it's more likely that it may have been underdosed.  If you have strong muscles there, you may just need more.  The muscles in different parts of the face require different amounts of Botox depending on the size of the muscle and how active they are.  For example, a man often requires 50-100% more Botox in his frown lines because the corrugator muscle is just larger.   Please call your doctor's office.  They may ask you to come in so they can see the problem.   Dr. I