I killed my first miltoniopsis (Miltoniopsis Bert Field 'Eileen') and I was so confused as to why it died. I was so careful about not overwatering and making sure that it got the perfect diffused light.
Turns out the key factor was not light or water, it was HEAT! It was simply too warm for it in my growing space in the summer, even though the space was air conditioned. I bought a new one (same exact plant) in the fall and it is still doing well. I put it on my first floor (instead of the 2nd floor) and will monitor it closely when things get warm again. I might even end up moving it to the basement in the summer.
So if yours gets grumpy and you have ruled out water and light problems - change the temperature.