They were right about the compost, so its not that, but if you bought it at a decent garden centre, it should be guaranteed for a year, so take it back and get another or something else.

Being potted means it is going to dry out at a rate or knots and with the very dry season to date i suspect you didn't give it enough soakings.

Once a week, fill the pot up to the top with water (use a hose or you'll be there for hours), let it drain down through and then give it another lot.
Did you give it this much water ?
If not, you have probably killed it yourself because it died of dehydration.

Did you site it in the sun ?
If so you killed it because it needs a damp cool site.

They do okay in full sun, Louise - as long as their roots are cool and moist.

I didn't know that Bamboo, always thought full sun was a no-no with them, thanks for the heads-up.

Early morning sun in winter is not a good idea, but I think, like a lot of 'shade loving' shrubs, really they like soil that doesn't completely dry out - which most often means a shady position.