In a reaction between Calcium Ammonium Nitrate or Urea with water, does Nitrogen Content and Purity affect how cold the solution gets?
The endothermic process is just the dissolution of the calcium ammonium nitrate (CAN) or urea in water.
The most likely explanation for the lack of cooling when you used your pack is that over time, by getting partially squished at some point, or from defective manufacturing, some water leaked out so that the CAN or urea had already been partially/fully dissolved by the time you tried to use it.
These reagents are pretty stable and it is very unlikely that the nitrogen had changed form or that the system had been contaminated.