It's normal for the pressure reading on your air pump to sometimes not change for the first couple minutes when inflating tires, especially larger ones. The pump needs to first build up internal pressure higher than the tire's existing PSI before it can start filling the tire. The pump has a one-way valve that prevents air from flowing back out of the tire until the pump pressure exceeds the tire's pressure. Once the pump reaches that threshold, you'll see the reading start to increase as the tire inflates. The initial lack of change is just the pump doing its job to pressurize before it can begin filling the tire.