Here you have some useful tips to help you increase your earnings:
 
EXPAND YOUR VAULT'S CAPACITY 
 
The first thing you have to do to increase your income is to level up the vault in your hospital. Don't forget to regularly check the vault and level it up until you get the max level. Moreover, check the Research list to run all the investigations related to this said room located on the bottom right part of your building. Remember: you can complete the game without spending real money. If you see a price higher than your money limit, there should be a missed level out there! Read carefully the pending requirements in the vault's dialog window.
 
UPGRADE YOUR ROOMS
 
All the upgrades you make in each room will affect your performance and efficiency, so you need to take care of every one of them so they can be armonized and balanced and can function properly with all department's help. You can check the upgrades or expansions available and their functions by tapping on each room.

Also, don't
forget about the delivery workers and room, as you may be producing a
bunch of supplies, but it will not be useful if you cannot deliver them
to the corresponding rooms! You need to check the whole medical process since step one.

2) Wanna increase your number of
patients? Check the rooms that have angry clients and upgrade
it as much as you can, so your prestige will rise and more patients
paying more will come.

DO ALL THE RESEARCH

The upper-right microscope icon will be your best ally from now on! All these sections will take some time to be developed, but they will help you expand your hospital. Check the boosters that they provide and choose the research according to your strategy. Some of them will increase your workers' slots and others will bring more improvements for your rooms, for example.

CHECK YOUR HIRED DOCTORS AND THEIR ATTRIBUTES

Take into account that your unassigned doctors will charge you anyway, so if you are not interested in some, review your list of doctors and consider firing them if they are not useful in these moments. This will reduce your expenses and help you continue with the game.

CHECK YOUR STATS

On the round graphic icon on the top right part of your screen, you can check with detail your economy. There, you can see if something is making you expend too much money or if you are not earning enough in some specific department.