Miltoniopsis pseudobulbs getting smaller

[Kaykay]

I have a young Miltoniopsis Anneville. Since I bought it in March with 1 backbulb and 1 pbulb it has put out 2 new growths. Which should be good news, but they are getting progressively smaller, which suggests to me that I'm doing something wrong. I've kept it outside over the summer, getting direct sunlight from sunrise to 11am, and then shade. It's watered with rainwater or deionised water (depending on rainfall!) and fed once a week with a high nitrogen feed. It has both high humidity and good air movement. It has put out new root growth, the pbulbs are all plump and leaves all healthy light green.
Any ideas as to problem or solution?
Thanks!
Kay
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