DIAGNOSIS This is classic lithium-ion battery wear, accelerated by how you charge it. Constantly draining it to zero or leaving it plugged in for days at 100% stresses the cells.
FIX To preserve what life it has left, change your charging habits. Unplug it as soon as the light turns solid green. Try to use it and recharge it more frequently, avoiding a full drain. For a temporary boost, run a cleaning cycle with warm water and a drop of dish soap to remove any gunk that makes the blades work harder.
IF THAT DIDN'T WORK The battery itself might be failing. There's no user-replaceable battery pack. Your options are to use it primarily plugged in with a USB-C power bank, or look into Ninja's warranty if it's less than a year old.
HEADS UP Blending ice or frozen fruit cuts battery life in half compared to soft fruits and liquids. For battery-intensive blends, consider having it plugged into a portable power bank.