I had two of these in pots and they quickly became pot-bound. You could check by turning the pot on its side and prising the plant and soil out. Three options are: root-prune; replant in a larger pot or in the ground.

or give it a good slow release feed. Mine is looking the same and a feed usually perks it up.

Thank you both. I shall try the feed first because it was only pruned and repotted year before last. It had been in the same pot for about five years without problem before that.