I purchased an '09 X5 3.0 brand new in Jan of 2009. A few months ago, the fuel gauge would only read 3/4 of a tank after fueling up full. I had the entire fuel sending unit replaced approx a month ago. The gauge read full capacity until recently, where the same problem arose once again.

Needs a new sending unit

power goes through the gauge down to the sender unit ,through the resistance to ground
what you have indicates a short in the wire after the gauge and before the sender unit

Loosen the pickup box mounting to frame bolts.Remove all driver side bolts and lift/tilt driver side of box high enough to access fuel pump sender unit easily.Remove fuel pump assembly unit and cycle float while watching fuel gauge.Should be as simple as adding a little more pressure on the fuel gauge sliding contacts on the fuel pump assembly.
This job should take about 1 hour.
Always lift the box when changing pick up fuel pump assembles as it is faster and easier than dropping fuel tanks.

Drop the gas tank and check for contamination/corrosion. You're going to have to do that to replace the fuel level sender anyway and check both the fuel pump (it's in the tank) and the primary filter on the pickup tube.

if it worked for a short period of time after doing the pump/sending unit the check the connections going to the pump/sending unit they would have been moved around doing the pump

a bad fuel level in the tank should not cause the tank not to fill up,but it can cause bad readings to the gauge. so the tank may have had more fuel in it than you thought and that is why it wouldn' t take the gas as it should. then when you filled up the next time the fuel level was correct so it filled up like you knew it was suppose to.I suggest you monitor the fuel with your trip odometer and fiil up every 200 miles to make sure the gauge is reading like it should or you may run out of gas.

My daughter's car does the exactly the same thing. Have had it in 4 times no luck. They say water in the gas and charge me for this and that. I Don't think the gas diagnosis is correct. We buy gas for 4 vehicles at the same place with no problems and 2 of the 4 are Kia's.

It sounds like the fuel sender is bad in the fuel tank.

You're either looking at a short in the wire that goes to the fuel sender, or a bad fuel sender. If it was just a broken wire, it would read empty instead. The fuel sender is a permanent part of the fuel pump, so that has to be replaced as a unit.

This is a normal concern.
The instrument cluster uses 4 different operating modes to calculate the fuel level:
Anti-slosh (default mode)
Key OFF fueling
Key ON fueling
Recovery
After a fuel fill up, the time for the fuel gauge to move from empty (E) to full (F) ranges from 2 seconds to 55 minutes depending on which operating mode the fuel gauge is in.
The default fuel gauge mode is called the anti-slosh mode. To prevent fuel gauge changes from fuel slosh (gauge instability due to changes in fuel sender readings caused by fuel moving around in the tank), the fuel gauge takes approximately 55 minutes to go from empty (E) to full (F).
The key OFF fueling mode (2 seconds to read empty [E] to full [F]) requires 3 conditions be met:
The key must be in the OFF position throughout the entire refueling of the vehicle.
At least 15% of the vehicle's fuel capacity must be added to the fuel tank.
The instrument cluster must receive a valid key ON fuel sender reading within 1 second of the key being put into the RUN position. The key ON sample readings are considered valid if the fuel sender reading is between 15 ohms ± 2 ohms and 160 ohms ± 4 ohms.
If these conditions are not met, the fuel gauge stays in the anti-slosh mode, which results in a slow to read full (F) event.
The key ON fueling mode (approximately 90 seconds to read empty [E] to full [F]) requires 3 conditions be met:
The transmission is in PARK (P).
The key is in the RUN position.
At least 15% of the vehicle's fuel capacity must be added to the fuel tank.
In key ON fueling mode, a 30-second timer activates after the transmission is put into the PARK (P) position (automatic transmissions). When the 30-second time has elapsed and at least 15% of the vehicle's fuel capacity has been added, the fuel gauge response time is 90 seconds to read from empty (E) to full (F). When the transmission is shifted out of PARK (P), the fuel gauge strategy reverts to the anti-slosh mode. The key on refueling mode prevents slow to read full events from happening if the customer refuels the vehicle with the key in the RUN position.
Recovery mode is incorporated into the instrument cluster strategy to recover from a missing fuel level input during a refueling event. Missing fuel level inputs result from intermittent opens in the fuel sender or its circuits. Recovery mode (empty [E] to full [F] approximately 20 minutes) is initiated when the following 2 conditions are met:
The instrument cluster is in the anti-slosh (default) mode.
The actual fuel level in the tank is greater than what is being displayed by the fuel gauge