If you get good hot water at the sinks, your water heaters are working properly. It sounds like there is a plumbing problem in the house -- the showers are not plumbed properly. If you get no hot water with the shower faucet on full-hot and the water stops flowing completely, check the shut-off valve for the shower faucet. (There may be a panel on the wall of the room on the other side of the shower. Open that panel and look for shut-off valves. All modern plumbing should have shut-off valves for all the fixtures.)
As for the volume of water flow, again, this sounds like a plumbing problem. Either your faucets have restrictors in them, or the plumber installed the wrong diameter pipes, or the shut-off valves are not fully open. It would be a real surprise if low flow of water was caused by a water heater -- unless you're trying to run several bathrooms simultaneously. The first thing to do is to track down all the shut-off valves and make sure they are fully open. Every sink, every shower/tub, every toilet, every faucet should have shut-off valves. Find them and make sure they are turned fully on.