For me it is the other way around, azathioprine (imuran) did little to keep me in remissiom at a dosage of 1.5mg per kg. A GI explained that to me 6 years ago. Azathioprine and 6mp work a bit like an inverse jump starter of an engine - if the jump starter is too weak, then the engine just doesn't start, or in other words even if say 3/4 of the normal jump starting power is used, the effect is still 0, the engine does not start. Similarly, the bone marrow suppression from azathioprine needs to suppress the white blood cell count to the lower limit of the normal range. A dosage that is too weak does little to actually suppress the immune system enough to "protect" patients from over reactions that lead to inflammation.
If you really have problems such as hair loss, have you considered to switch to biologics?