The self-contained 40MAX Grenade Canisters are pretty simple and reliable, but they do require maintenance like most performance foam blasters.
Clean any dirt or debris that may get on the inside of the canisters. The canisters can be easily unscrewed into two halves.
Make sure the o-rings are clean and lubricated well. Use non-petroleum or non-aerosol based lubricants. Use something silicone based.
For the 40MAX to refill correctly, the front and rear buttons must be fully reset against the front and rear faces. The rear button can be pushed in fully to reset the front button.