I have that problem at the minute - I am going to leave it for a couple of days out of the filter (next to it) as it might spread the heat more - and if it does not walm it up I would advise to either turn it up/get a new heater/more powerful one.
i have turned it up for now just to see if it makes a difference, might take it out of the filter like you have for a bit and see if it helps.