I'm no bamboo expert but i do grow grass(turf) for a living. I would overstrurate the soil. Put a hose on it for a couple of hours. Pronto.

And next time, use slow release fertilizer, like Osmocote or equivalent.

Your bamboo is probably fine. You have to work pretty hard to overfertilize a Phyllostachys. Most likely those two smaller shoots were going to abort anyway, since Phyllostachys usually start more shoots than they can mature. The extra shoots are the plant's form of insurance. If some of the shoots get damaged/eaten/harvested by weather/animals/people, then more of the remaining shoots would grow to maturity rather than abort. You have to remember that in Asia, Phyllostachys evolved along with a host of bamboo-specific insect, animal, fungal, and bacteria that target bamboo shoots. So the plant "expects" to lose a certain proportion of shoots to these organisms. But when black bamboo was imported to North America, it left most of these pests behind, so it initiates more shoots than it can mature when left unmolested.

I guess I get molested alot, both years now I have lost half the new growth coming out of both my Phyllostachys, cant figure out how to keep them all.. I overwater, I fert more but still lose the same amount as the year before.. also my ph.a.a.f didnt double in size this year at all, stayed the same size as the year before so a bit disappointed. My dulcis increased a bit, got a few one inch culms..