Marketing, screening, collecting rent, unpaid rent, maintenance, capex, insurance, lawn care, vacancies, turnover costs, inspections, and so much more. 

Knowledge is power:Thorough Inspections: Minimize surprises with detailed inspections and due diligence.Strong Screening: Careful tenant selection reduces vacancy and headaches.Diversification: Spread your investments across different locations and property types to mitigate risk.Financial Planning: Maintain a buffer and choose financing options that fit your long-term goals.

The rental demand here is consistently strong, so vacancies are rare and turnover is quick.

Last quarter the vacancy rate for apartments was below 3%.

Significantly more negative when including vacancy, maintenance/cap ex, pm, misc.   

i.e. longer than normal vacancy, bad tenant screening, poor maintenance, skipped rent payments, etc.There could be a handful of reasons why it lost it's ability to cash flow.

Any home you buy with $20,000 is likely going to be a fixer-upper, which means it can be heavy on repairs, turnover costs, vacancies, etc.

In areas of sub 3% vacancy rates.

Most times landlords don't have data or cannot adequately factor in vacancy time or turnover rate.

Cities in the San Francisco Bay Area are charging vacancy taxes because they're seeing people with a lot of capital just buy up properties to hold onto for appreciation and not renting them out.