I have had Nitrous before on previous cars and Josh has had Nitrous on his Si for quite sometime so the rookie mistake of the bottle being empty is not a possibility.

He was purging fine and purging strong...

My diagnosis of why he wasn't running faster.

1. Bad ZEX nitrous solenoid, not the purge solenoid.
2. Bad compression/tired K20 motor that may need overhaul
3. Maybe it needs a Retune.
4. last but not least, Driver mod lol. However Robert did drive Josh's car and still got spanked when Josh drove Roberts car.
5. Lets see John drive Josh's car down the track lol!!

Bottom line in my opinion is.. go boost, and stay away from the cheap nitrous horsepower.