We were having poor cleaning on the upper shelf of our Beko DWD8650X. Opening the door midway through the cycle showed that the top arm is not spinning (but water jets were coming out). I cleaned the dishwasher thoroughly and checked the impeller (nothing to note).
Suspect that top basket is not going back far enough & not seating correctly.
Theres a (grey) flat pipe coming up back inner of dishwasher called feed pipe, it has 2 wings/guides, if one or both of these are bent it will prevent the top basket going back far enough to locate properly & poor water feed to top spray arm.