Small bamboo bought July, is mostly yellow

Hi, 

Looking for a hardy perennial, I this year bought an 18 inch healthy looking little bamboo with about 20 mostly green fronds, with the garden-centre's advice it was a type that would be happy in a pot (sorry I dont know the name).  I potted it in potting compost into a new pot about 20 inches tall, and have watered it regularly when needed, and put it in a sunny and sheltered corner of my little garden. But after 6 weeks it was looking yellow (few green fronds now) so thinking it was restricted I bought it a very tall pot, about 3 foot high and filled it with potting compost, and potted it up again.  I was surprised to find the roots had not expanded at all from it's original little pot-size.  But they looked healthy.

Anyway, this is now about 3 weeks later again, and despite lots of sun, and care, the plant still looks poorly in its big pot.  It has perhaps 2  or 3 tiny green fronds appearing but that's all.

Please, does anyone have advice about what I'm doing wrong?