It is difficult to know what you are doing wrong ; but Miltoniopsis are not strong plants - they won't take the kind of treatment which Cymbidiums will stand for example. I suppose these plants have come from one of the big production units in The Netherlands - were they fresh and good when you got them ? If they had been in a shop for some time and got dried out, it would give them such a set-back as they would be unlikely to recover . They are never worth buying "cheap because the flowers are going over" - the plants will be too !
The compost is probably almost soil - is it dry or wet ? They could do with being absolutely soaked, and then allowed to get almost dry before being watered again.