perhaps it may be a lack of water as it is in a pot. That is more likely than lack of food. Make sure it has plenty of water and don't leave it in a windy spot. Has it become pot bound? if yes then repot into a larger container.

Alas, it is far too big for me to re-pot. I do water regularly but would like to find, probably, better food for it

Thank you

Not sure its a good idea to feed in the winter. Perhaps it will green up now the growing season is approaching.

Actually I meant Spring feeding, since there will be very little nutrition left in the tub in which it has spent all its life, so far?