DIAGNOSIS Weak coffee usually means the water is going through the grounds too fast. On this Ninja, the most likely culprit is the 'Rich' vs. 'Classic' brew style selector being on the wrong setting, or a clogged spray head that's not distributing water evenly.
FIX First, check the brew style dial on the front. For a stronger cup, make sure it's set to 'Rich'. This slows down the brew cycle. Second, remove the brew basket and look up at the shower head (the part that sprays water). Use a toothpick or a small brush to clear any coffee grounds or mineral deposits from the tiny holes. An obstructed spray head dumps water in one spot, leading to under-extraction.
IF THAT DIDN'T WORK You might be under-filling the water reservoir. The 'Scoop' guide on the side is just a suggestion. Try adding an extra 4-6 ounces of water to your usual amount of grounds to increase saturation time. Also, make sure you're using a medium grind—too fine can clog, too coarse lets water rush through.
HEADS UP The 'Rich' setting adds several minutes to the brew time. That's normal. Don't switch it back to 'Classic' thinking it's broken because it's taking longer.