I live in an apt & on a boiler for hot water. My building's water doesn't heat up good. How do I fix it?

I'm the manager of the complex and have had maintenance check the valves and called a plumber who says they have no idea. We are the building farthest from the boiler and they water doesn't get hot especially in the morning. Later in the day if you let it run for at least 20 minutes it does get a bit hot but not really hot. Any ideas for me to why this is happening. I have an unhappy tenant b/c this has been going on for months and no one knows how to fix it.