Have been trying to work out why our dishwasher is no longer cleaning
properly (a 5-yr-old Bosch built-in; can't find a model number).
Basically, stuff on the top rack, and sometimes on the bottom one,
invariably comes out with gritty residue stuck all over it. Things I've
tried out/checked:

- Ensured spray holes in rotor arms aren't blocked;
- Cleaned filter in bottom of machine;
- Opened door mid-cycle to check rotor arms move (but can't tell how
vigourously though)
- Changed detergent (we're in a soft water area so no need for softener)

During the wash, water reaches the upper rotor via a flat grey conduit
attached to the inside of the machine, and which mates up at the back of
the upper rack with an aperture leading to the upper rotor; I'm guessing
that for whatever reason not enough water is getting through? How can I
tell?

One thing I've noticed is that after a cycle, if you lift the filter the
water in the machine's sump is really mucky; is that normal or should it
be basically clean 'rinse' water down there?

Any advice much appreciated!

David