Maybe your nutrients are low.

I keep cheato in my refugium. I don't have fish, I don't feed my tank and I don't add supplements besides C, Alk and Mag. My cheato barely grows but it hasn't died. It could take a year before I could even have a gold ball to tennis ball size of cheato to harvest.

If your tank is devoid of nutrients then the cheato isn't going to grow (possibly die back?).

Christopher