Heater temperature does not stay hot

Liquids in general contract with cold temperatures and expand with hot temperatures. This applies to gasoline, too. However, it's a myth that filling up at a certain time of day makes a difference to the volume of usable fuel that you pump.----------------------------------------------------------------------------------
VAG Dash COM V1.65

1st thing to check is the coolant level. I've seen cars w/ no heat, or very little quite a few times caused by low level in cooling system--not low enough to overheat the engine, but enough that circulation of coolant to heater core is affected (no or little hot coolant circulation to heater core=no or little heat),
Is the engine temp gauge in the normal range? If it's in the low range, engine not reaching operating temp. it could be caused by a thermostat stuck in the open position (or no thermostat, sometimes people remove them when their car overheats, and this should not be done).
Here's how the system works: the heater 'box' contains a heater core (a mini radiator) w/ a fan and hoses connecting the heater core to the engine cooling system. Hot engine coolant is circulated thru the heater core. When u activate the heat switch on the dash, a flap door opens to expose the heater box to the passenger compartment. The fan switch on the dash activates the fan, which blows air thru the core, which heats the air.
So, if it's not the coolant level at the radiator, the fan is obviously working, the engine is reaching operating temperature, I would say it must be the flap door that opens, isn't opening fully, or isn't staying open. It could be the dash switch is defective (not supplying current to open flap door) or less likely the flap door is binding somehow. Hope it's as simple as correcting coolant level, if not, the dash switch control should be checked