I assume the 120mg Mestinon is timespan? Are your also taking Mestinon during your waking hours? They also have 180mg Mestinon timespan tablets too.

The MG Manual has a great section on Cholinesterase Inhibitors and goes into detail about the use of Mestinon:

No fixed dosage schedule suits all patients. The need for cholinesterase inhibitors varies from day to day and during the same day. Different muscles respond differently—with any dose, some muscles get stronger, others do not change and still others become weaker. The drug schedule should be titrated to produce an optimal response in muscles causing the greatest isability.

Myasthenia Gravis: A Manual for the Health Care Provider (PDF) - page 17

The PDF manual is free. You can download and print a copy for yourself.

Take care and keep us posted,

-Mark-