DIAGNOSIS This is a technique issue, not a machine failure. The dough program is designed for specific types of dough, and if the hydration is too low (too dry), the blades can't grab and knead it—they just spin a hard ball around.
FIX For bread dough, you need a softer, stickier starting mix. Try this: 1) Start with all your liquids (water, milk, eggs) in the jar first. 2) Add all your dry ingredients on top. 3) Secure the lid. 4) Select the 'Dough' program. The initial mixing phase will pull the dry into the wet from the bottom up, creating a better mix. The total flour should not exceed about 4 cups of all-purpose for this jar size.
IF THAT DIDN'T WORK For very dense doughs (like whole wheat or bagel dough), the Ninja might struggle. You may need to stop the cycle halfway, scrape down the sides with a spatula (unplug it first!), and then restart the program. Or, mix it in two smaller batches.
HEADS UP The 'Dough' setting runs for a set period. Don't try to use the Pulse function for dough—it won't give the continuous kneading action needed. Also, the motor can get quite hot running this program, so give it a long cool-down period before using it again.