dishwasher does not wash well

Dishwasher does not wash well. Occassionally there are soap suds, or undissolved soap granules remaining after cycle is complete. The dishes do not clean well. Have tried several different dish detergents, and checked all basic components (spray arms, filters, etc). Any ideas? Do you think running diagnostics would be helpful? Do you know anything about running a "cleaner" through the dishwasher? Have downloaded troubleshooting manual. Not sure what to try next? Thanks.