My zebra grass has no stripes!

[Dozy]

Can't understand it, I had the zebra grass in a pot in the back garden for years and it always came up stripey.

 

It went in the front garden 2 years ago, and I definitely had stripes for the last 2 years, and has flourished - it had wonderful seed heads (is that the right term?) last year.

 

But this year, only one of the shoots is striped - all the others are plain green. I haven't done anything to it - I haven't added anything or changed its habitat at all, so why has it suddenly turned plain. And is there anything I can do to make it go stripey again (apart from going out with a tin of paint and little paintbrush!!).

 

Any help gratefully received.

 

Dozy