whirlpool dishwasher soap door doesn't open

Hi,
I’m new here & in hopes someone can help me figure out my problem. I have a model DU8500XX3 dishwasher that the soap door isn’t opening during the cycle. It goes thru the entire cycle ok as far as I can tell. I took the door apart & checked the door mech & all seems good, so what tells the door to open when needed? What should I check for? It’s an older washer & some parts are NLA but I really don’t have the funds to replace the washer…any help would be great!

Thanks,
John Young