is it root bound? its something to do with the roots.

I'm afraid I dont know what that means - can you elaborate?

Here in Vegas we have extremely rocky, hard, alkaline clay. Obviously I mixed in good soil in the hole when I planted it, but is it possible if the hole wasnt large enough that its roots arent expanding past the dug hole?

It is normal for alot of bamboos to shed.Especially if you have cold weather.You are in the desert.So you have cold nights.It will come back.Remember this,bamboo:it sleeps,it creeps,it sprawls!

Have to wonder about climate suitability. You have seen other plantings there?

Yes; they need a bit of water, but bamboo is moderately common in Vegas

We use Giant Timber Bamboo here in Phoenix, similar climate, does OK with enough water. Shading of the lower canes/root ball helps. It's not "can't screw up" type plant here. You have to get a few things right, but it will forgive a few mistakes too.

We have almost concrete hard clay soil here...when dry. When damp it's very easy to dig and roots to move thru it no problem. Most people see the concrete hard dry stuff and think it must be amended...but it doesn't. In our climate, with < 10% humidity and 100+F temps clay soil is great because it holds moisture. Amended soil can mean more frequent watering. Much different than many places in the world.

@others...not all deserts have cool/cold nights. We sure don't.

It could just be spending its time producing roots like secretsquirrel007

said. FWIW I planted Giant Timber Bamboo last fall and it's nice and green, not loosing leaves. However it did stop growing at least above ground. It was sending up a new shoot when planted and that stopped. Pretty normal in my experience. I expect new shoots next spring.

It's worthwhile to dig around a bit in the soil around the plant. Soggy wet? Dry? Easy enough to check and kind of first thing to check. Drip systems sometimes clog. Sometimes an emitter blows off and you get too much water.

When first planted some people put the drip out away from the plant where the roots will be someday, leaving the root ball dry. Might be OK in most climates...not ours, except maybe winter. Now is when that becomes a problem. IMO, for our climate, I like a water pan type deal that can be filled with 1-2" of water per day at least for the first year, Probably around 10-20 gals per day. How much water are your emitters putting out?