could just be unhappy...mine did same thing. For first 2 months in tank would get good PE seemed to be growing, then all of the sudden wouldn't open up all the way, now it is moved into my 40 with more light and lil more flow seems to be rebounding. It has never been as big as my torch and frogspawn though.
You might try moving it just a couple inches over or up/down for abit see how it reacts.