Onc. Mendenhall Hildos leaves not mottled anymore

[CourtneyT]

I got this beautiful Onc. mendenhall Hildos mid last year and it bloomed twice with gorgeous flowers. However, I observed the fading of the red line pattern on the leaves within the past few months. The leaves are just green now. Also, it seems not forming the third flower bud either. I checked the root and it does not look too bad. I have been growing it near the north facing window with LED growing light on from 5pm to midnight everyday since November. The humidity is about 60%-70%. Temprature is between 58 and 70. I water it once a week. Any suggestions about the caring? What could happen with the leaves and the bud?