It looks like it’s got lots of new growth at the top, but it’s the lower leaves are dying off. I’d prune off some of the lower leaves, so it’s not wasting energy on them. It’s winter, so I wouldn’t expect it to be as active now as it would be in spring.
How much light does it get? Can you move it to be closer to the window? Also it likes humidity, so either keep it in the bathroom, or mist it a couple of times a month.
Pruning should stimulate some new growth, but I wouldn’t go crazy - just thin out some of the dead looking ones. You can always repeat in March if you need to. Crotons are fussy buggers, I wouldn’t take it personally!