I struggle with this too. I have a long-term tenant, who is paying below market rates. I am hesitant to increase the rent. The family pays on time, and rarely makes requests. It is profitable and on autopilot. Everything I want.

Raising the rent creates additional cash-flow to apply to the principle on the mortgage, so in my self-interest to do so.

The tenant has been there 4 years, and we have not adjusted the rent. 1 month of lost rent may take 2 years to break-even. You also have the risk of choosing unwisely with the new tenant – who could leave in a year after the lease is up.