The ref didn't know what he was talking about, most likely.  I've struggled with HPA for a while on a stock 98c and have called Tippmann, Palmer Pursuit and Bob Long's shop. 

First of all, CO2 is a much denser gas and will carry some liquid with it, into the valve, regardless of having an x-chamber or not.  (However, the liquid I'm talking about is particle liquid carried as vapor, but not yet gas.)  Because of that, some of the CO2 will still be expanding (changing state) in the valve, thus more pressure.  With N2 or HPA, there is no state change (thus very consistant).  However, it is not uncommon at all to have to increase the pressure on your reg to get your gun to get proper velocity, or even recock.  Most high pressure HPA tanks only really deliver about 780 psi, some even lower.  A stock Tippmann is supposed to use a minimum of 800psi just to recock.  Although, I've seen them run as low as 680psi on CO2, but remember, there's still some particles that will expand in the valve to develop the proper recocking pressure.  When using HPA, the pressure out of the tank is what the valve will see...PERIOD...Although, most Tippmann don't have a problem running on HPA, as stock, it isn't uncommon to have the problem you're talking about, this is why Tippmann developed the low pressure kit for their markers (as told by Tippmann themselves).   Because HPA tanks typically run at 780psi or lower, some stock Tippmann can't develop enough blowback to recock the bolt. 

You know what, I'm ramblin'.  Just know, that what you experienced isn't uncommon and if it worked fine on CO2, I doubt there' anything wrong with your marker.  You might resign yourself to stick with CO2 or get and LPK and a reg.

Again, don't flame me, as I know the Tippmanns should run fine on HPA as stock, but it has been known by Tippmann, Palmer Pursuit and Bob Long that some will not run on HPA (depending on the tank, also).