AC is relatively weak in JDM cars specially 660cc Kei cars. I have not experienced N-WGN more than once or twice, but I have seen this in other 660cc cars, the AC is generally not strong, and it turns off when the car is stopped or if the accelerator is pushed hard.
The way forward at this time is to get the AC serviced and make sure that it is mechanically ok and gas is filled as per requirement. This should improve the situation if the AC is performing sub-optimally for some reason. But after that, most JDM Kei-cars have weak ACs designed to give priority to saving fuel.
There are local solutions available such as replacing with local Alto AC, or turning off the cut-off function when car comes to a stop. But if you choose this route please proceed with extreme caution since it can damage the car or cause troubles.