DIAGNOSIS The drive socket on the base is not engaging the blade assembly's coupler. This is typically caused by a worn or stripped plastic drive coupler on the blade assembly.
FIX Inspect the blade assembly. Remove it from the cup. Look at the black plastic hexagonal coupler that inserts into the base. If the edges are rounded, chewed up, or cracked, the coupler is stripped. You must replace the specific blade assembly (part NN1101 for the 16oz cups). Order the official replacement. Do not use generic blades.
IF THAT DIDN'T WORK Examine the metal drive socket inside the base for debris like frozen fruit or ice chips. Unplug the base. Use a damp cloth to wipe out the socket. Ensure no foreign material is obstructing the coupler from seating fully.
HEADS UP This wear is accelerated by over-tightening the blade assembly onto the cup. Hand-tighten only. Never use tools. Also, ensure you are using the correct blade assembly for the 16oz Fit cups, not from a different Ninja series.