Plumbers/HVAC Techs, What usually makes invoicing or collecting payment harder than it should be?

I grew up around plumbing. My dad owns a company and my younger brother’s in the field now. One thing I hear come up a lot is how frustrating it can be to finish a job and then deal with actually getting paid.

If you’re a plumber/HVAC Tech or run a plumbing/HVAC business, what tends to make that part harder than it should be? Is it chasing down homeowners, unclear pricing, awkward conversations, or something else?