Can nitrous oxide be ineffective in certain circumstances?

My question is: Can nitrous oxide be ineffective in certain circumstances?  
     I had a dentist appointment this morning, and I always use the nitrous oxide because of severe anxiety.  Before the procedure the dentist did everything as normal, however, I never felt the effects of the nitrous.  I remained completely "normal".  When asked if I was okay I told them the nitrous wasn't working and they reassured me that it was on and was floating the ball in the hose at a 50/50 mix.  I knew it wasn't working right and halfway into the operation I suffered a strong panic attack.  I still believe the tank was empty, but they informed me that sometimes nitrous oxide does not work because of our emotional state when receiving it.  Again, I've had nitrous oxide at least 15 times in my life, and it has always affected me precisely the same way, except for this time.  I'm 28 years old and I cried all the way home.  Any answers would be helpful, and if I didn't state a detail feel free to ask.
~A~

[braveheart226]

  If you were a patient in my office and remarked that you weren't feeling the effects of the nitrous, my doctor would have raised it. I fully believe you when you say that you didn't feel the effects this time. You'll never get a straight answer if the tank was, indeed, empty.
   If this is a deal breaker for you, there's no reason why you can't find a new dentist.

         regards, Sue

[adaminchrist]

Thanks Sue!