DIAGNOSIS This is almost always a water supply issue. The pump isn't pulling enough water from the reservoir, either because the intake tube is kinked or there's an airlock in the system.
FIX First, check the clear silicone tube that runs from the bottom of the reservoir up into the machine. Make sure it's not pinched or twisted. If that looks good, do a system prime: 1) Fill the reservoir to MAX. 2) Hold the brew button for the Hot XL size. 3) Keep holding it until you hear a steady stream of water hitting the carafe, then you can let go. This forces the pump to pull water through and clear any air bubbles.
IF THAT DIDN'T WORK The internal water filter might be clogged with scale or sediment. If you use the Ninja water filter, try removing it and running a brew cycle without it. If the flow returns to normal, you need a new filter.
HEADS UP Always use the reservoir's handle to slide it straight in and out. Yanking it sideways is the main cause of that intake tube getting kinked.