Does this ring true?
"It appears a domestic, storage type of hot water heater was replaced with a tankless, on-demand type of water heater. If the water supply piping about the house was originally configured to accommodate a storage type of hot water heater, and the new water heater is utilizing this original piping layout, you may find that an adequate amount of hot water will not be present in areas of the house when multiple water fixtures are in use at the same time. If this turns out to be the case—and only time will tell if it will—then you may need to replace this tankless water heater with a conventional, storage type of hot water heater."