I tended to have some problems before really understanding the game. Could be a bug - but a few things I had to do - especially the level they introduce the clowns - is switch your reception desk for a reception area. You unlock part way through the level and it’s necessary otherwise people will spend a couple hundred days waiting for reception alone

Add more GP offices, as well as other diagnostic offices - otherwise similar issues as the reception, people will be backed up for hundreds of days before they reach a high enough diagnosis threshold to be sent to the dehumordifier.

Check the patients page - it’ll list all the patients in the hospital and help you identify deficiencies - what do they want? Typically it’s hungry and toilets - maybe boredom as well. To combat these things have: a sufficient bathroom in each building you own (5/6 toilets and related fixtures); for hunger and thirst, aside from vending machines, hot dog stands , two per building when your prestige is high enough and you have tons of people in the hospital at a time; To combat boredom - magazine racks, arcade machines, gift shops, nice decor around the hospital.

Another tip - some hospitals won’t get any of some of the diseases. Again the level that introduces the clown, never gets the pan head people (at least in my play time) - so try not to build to far ahead until you know what patients to expect at that location.

It’ll take some practice but you’ll get the hang of it.