It could be any number of things: 1) Low nutrients, 2) low light (I know you said the light got good reviews but keep reading) 3) wrong nutrients, 4) out competed by things in the DT 5) lack of flow 6) when thick enough, the cheato on the bottom of the ball tends to get blocked from the light b/c the ball gets too dense. There are probably many other reasons, some of them unknown as well. Anyhow, I had the same experience. When I first put cheato in the sump, it grew quite well. After a couple of weeks, it needed to be cut back. However, a few weeks or months later, it started to slowly die off. It never went away completely but at one point it was about 90% gone and I almost gave up.
I tried the power compacts, LED purplish grow lights, and blue LED's from amazon but nothing seemed to help much. Eventually, I stumbled across a thread about building a box out of egg crate and putting the cheato it in. I liked the idea so I tried it. It made it easy to trim back and helped keep cheato from flowing all over the sump. It seemed to help a bit. My guess is that the egg crate helped b/c it got it really close to the surface of the water and off the bottom of the fuge. It was now closer to the light source. The deeper the cheato lies, the more light is absorbed/reflected away from it by the water. Seeing this improvement in growth encouraged me to up my light power so I added a 2nd bulb (one of my grow leds) to my power compact. This seemed to help a little bit. A few weeks ago, I purchased a couple of GHL Mitras lights for my DT to replace my Kessil 360WE's. Since I had a couple 360we's laying around, I used one of them to replace the 2 bulbs that I was using in the sump. BOOM! The cheato has now been exploding in growth. It really made a huge difference. BRS has a video on their kessil h380 and it's impact on growing macro algae. I recommend it even though I'm using a slightly different albeit as powerful a light.
So in summary, I think pulling it off the bottom of the sump, adding a TON more light, and patience will do the trick.