Kangaroo paw yellowing

Hi everyone!

Anyone know why purple kangaroo paws have started yellowing? My newest planted one (Aug/Sept, rest planted in May) is the worst and then the next closest to it, and then the one that’s still going great is furthest from it.
New flowers are affected too.

I gave a seasol drinking a couple weeks ago to see if that would help and cut back some of the yellowing flower stems.

Noticed it started when I went away for 10 days and garden was mulched in that time (end of Sept).

3 photos of yellow and 1 photo of thriving flower.

Thank you! 😊