There are many mistakes landlords here is a Top of the Pile List
1 Overpaying for a property. ( Having a relationship with the property and not the Numbers) 2 Not hiring a property manager and thinking you can do it yourself. ( Bad rookie mistake) 3 Improperly screening the tenants. ( Just a killer getting the wrong person in) 4 Being lenient with a tenant. (They are not your friends) 5 Not charging late fees( This means they will pay late always) 6 Not responding to phone calls. ( If you do not care neither will they) 7 Not repairing problems correctly. ( This means you are going back) 8 Only visiting the property when there is a problem. ( Your missing the game) 9 Being lazy with your investment. (Tomorrow means I am okay with losing today) 10 Not having the gumption to evict. ( They will stay and not pay as long as you let them)
These are a few but there are many more. Hire a Property Manager.