2000 Chevy S-10 Smog emissions ( nox )

Hi, I just recently failed smog. I pass at the 25 mile an hour part with flying colors. But at low rpm my nox is thru the roof. I replaced the cat, lower exhaust o2 sensor and it still fails. Aside from replacing upper exhaust o2 sensor, what else can I do????? Your help is greatly appreciated and much needed. Ty