DIAGNOSIS The vacuum is laying down dirty water instead of picking it all up. This usually means one of two things: the roller is oversaturated with dirty water, or the suction path is partially blocked.
FIX Stop and empty the dirty water tank immediately—even if it's not full. Do a manual clean of the roller (pull it out, rinse it, let it dry). While it's out, check the suction inlet right behind it for clogs. Reinstall a dry roller and start fresh with clean water in the clean tank.
IF THAT DIDN'T WORK The other culprit is a clogged or wet filter. Remove the filter from the dirty water tank (the cylindrical one). Rinse it thoroughly under a tap until the water runs clear, then let it air dry for a full 24 hours. A damp filter kills suction.
HEADS UP Don't use cleaning solutions or vinegar in the clean water tank unless Tineco specifically says you can for your model. It can damage the pump and sensors. For the S5, stick with plain water.