Both hoses should be similar in temperature. If one hose is considerably cooler then the heater is restricted. Flushing the heater works sometimes but replacement is often necessary. Depending on the vehicle, the temperature control lever may control a water valve in series with the heater core, or an air door that mixes hot and cold air. The lever may operate via wire cable or vacuum controls.
Water pumps can have insufficient flow.
Restriction in heater hose connections at the intake or water pump will slow the flow through the heater. The antifreeze - water mixture can effect heat transfer. Heater cores on some cars can be difficult to replace, taking your vehicle to a good radiator repair shop may be in order. These tips courtesy of . Repairing, Recoring, Industrial, Commercial radiators.