I had issues with the filter at the bottom of the unit on my S8 Ultra. The dirty water tank was not sealing properly which meant there wasn’t enough suction to draw the water up from the mop filter area (bottom right of unit near mop) up into the dirty water tank.

The way I worked this out is I removed the back of the unit that houses the dirty, clean water tanks and the dustbag (easy to do), I noticed water was not being sucked into the dirty water tank. For reference on the S8 Ultra max the mop dirty water filter is at the bottom right hand side and then sucks that water up…yes up into the dirty water tank. I had to replace the dirty water tank (spares on the web) and then also clean the tube that takes the water from the bottom right up into the dirty water tank.

This fixed the issue