Here are my thoughts if anyone else wants to weigh in. Based on your nitrate/phosphate levels, I don't think it's a nutrient issue. Based on your WC schedule, I doubt it's a lack of iron, manganese, or any of the other trace elements (I doubt it will harm anything if you would like to dose a commerical mix of trace elements to see if you notice any improvement, but I wouldn't necessarily go out of my way to buy a new bottle).
Therefore, I'm going to guess it's one of three possibilities. 1) Your starting ball of cheato is too small (this is possible and will contribute, but isn't what I would start with), 2) Your light isn't adequate (Probably where I would start), or 3) your cheato doesn't like your flow (least likely in my opinion, cheato will tolerate a WIDE range of flows).
Therefore, I would probably look at upgrading your light and seeing if you notice an improvement there. There are a wide variety of lights that will grow cheato. I'm not familiar with your particular light, so I can't comment on whether it's good enough or not, but it seems the least expensive to swap out and see if that works. There are quite a few lights people have had success with in the $15-25 range or you could go with any of the nicer fuge lights like the cheatomax or the kessils. It's also possible it's a combination of cheato ball size and light.
Those are my thoughts in case anyone wants to add anything