I used to have this problem, and found it to be caused by a combination of two factors:

1) Growing up in a house with low water pressure. If the machine doesn't have much pressure to work with, it seems to do a bad job more or less regardless of what you try to do

2) Given you have sufficient water pressure, using cheap detergent. After switching to a "top of the line" dishwasher detergent packet, a dishwasher went from "borderline useless" to "more clean than I can get them by hand".

Number 2 was by _far_ the most impactful thing I've done; I can essentially now scrape dishes of large food chunks then put them straight in the dishwasher, and have a >= 98% confidence they'll come out completely clean.