Even though they don't feel cold, they are still working. If your ThermApparel Cooling Packs are still in a semi-solid state (meaning you can’t see through the entire pack), they are still pulling heat from your core and cooling you down. Feel the skin on your back. Is it cold? If not you might have the wrong size and need to do an exchange.
The main reason why your vest may not feel "cold" is because ThermApparel Cooling Packs melt and freeze at 70F / 21C. Example: Think of jumping in a swimming pool with a water temperature of 70F / 21C. At first, it will be frigid, but after swimming for a little while, your body will adjust, and it doesn't feel as cold, but the water temperature has not changed. It also has to do with the amount of coolness. Think of washing your hands in water at 70F / 21C. Now think of taking a shower with water at 70F / 21C. Burr!
Try the same activity you were doing with and without your ThermApparel UnderCoolCooling Vest and take notice of the differences. Compare both your fatigue and cog-fog levels. After you take the vest off, touch your back. Is it cool? Does the cooling effect last a while even though you are not wearing the vest?