What might cause an intermittent issue like that?
Explaining what was said earlier.
If there's no recirculating line (or even a pump), which there probably isn't since this was not the originally planned for configuration, then davka in low usage times you will have an issue.
If tenant #1 takes a shower at 10 pm, then at 11 pm water will be hot in the pipes and won't have to wait. If no one uses the water all day and 6 am the next morning tenant #2, who is far from the water heater, decides to take a quick shower, can take 10-15 minutes to get hot in his shower.
Another issue could be a faulty single handle valve which allows cold water to cross over into the hot line, thereby precluding the hot water from ever getting properly hot.
A good plumber can fix #1 and test for #2