Muscle Spasms After Botox Between Brows

Yes it is possible to dilute the Botox to the point of no effect. However, it is also not realistic to achieve complete paralysis of the muscles. If this is your goal, you will risk causing eyelid drooping due to overcorrection.
However, 6 weeks is too early for the Botox to be wearing off. Discuss your goals with your physician

I have recently had a patient with the same problem after her first treatment in the same area, but with Dysport, not Botox.  I have been treating patients with Botox for over 15 years and have never seen a spasm like this before.   I do not think additional Botox (or Dysport) would work, if there is no significant movement when you try to mimic the movement, as this means that the treatment was effective in blocking the nerve’s ability to cause muscle contraction (this is how Botox and Dysport work).   The muscles are apparently going into spasm without receiving any nerve signal to contract.  Spasms like this are indeed called involuntary muscle spasms, and are more common in larger muscles than in facial muscles.   I would be interested to know if this was your very first treatment with Botox, and what has happened to the symptoms since you first posted this question on RealSelf.
 

At times muscle spasms are due to remaining function in the muscle which could be corrected by injecting more botox to completely paralize the muscles.  You may consider seeing your doctor for evaluation.