When I checked the bath that have WD issue I found a lot of big sediment (see in picture) then I manual stir until disappear.
after that I top up level by water and wait for good temp. When the temp. reach 94C I let operator dip lenses and... they found WD defect again!!
This is my experience, sorry too long and if you not understand please contact me.
Finally thank you to you to try understand our situation kha.