From what I have read...it does seem that loss of efficacy is from one of two issues. One being longevity...which is not your situation.
The other being an unreached therapeutic level as you have already guessed. The members that experienced the same, and increased, did at last find their sweet spot in dosage...and went on to enjoy the benefits of their Rx.
So keep hopeful that this last increase will be the one that finds you just where you need to be!
Keep us updated, so we can keep you encouraged.
"Courage is not the abscence of fear, it is feeling afraid and doing it anyway!"
"Worrying does not empty tomorrow of its troubles...it empties today of its strength."
Corrie Ten Boom
Panic Syndrome recovery due to CBT