Do you have a light over the sump? I had the same problem for months until i changed the light bulb over my sump. I put in a 5100K PC floodlight 18 or 19 watts with 85watt equiv. and it's growing really fast. I think the original light i had was 2700K from HD. It's special order item at home depot but i think you can get it online.. i have the site somewhere. PM me if you want the link.
Well I have the same problem, but what I am seeing is that cheato takes time to grow.
If you just put them in the fuge, its will be dirty and lot of dirt on it. Just leave it or clean it if you want too, about 2-4 weeks, when all the little guys ( fuge animals ) started growing, the cheato will be clean by it self and growing. I have 3 HOB fuges.
I killed 3 batches of cheato before I found one setup that worked.
The 65W 6500 flood light did not work for me, I think it was too much light and cooked the algae into spaghetti. Raising it higher did not work for me.
Tried a 13W 10k PC light and a 25W fluorescent bulb and nothing happened for 2 weeks, the chaeto eventually started to break apart. Then finally got a "daylight" bulb from walmart, I think it is 13W. That finally seemed to do the trick. I have to trim the algae almost every 2 weeks now
I don't undstand why growing cheato is so difficult for some people. I think your tanks are too clean
I routinely give it away and all I have it is in my sump with a 100w bulb over it 24-7 and within a couple weeks it fills my sump.