It can be only one of two things:

1) Something in the car is draining the battery. *Someone* has to disconnect a battery cable, insert a meter in line, and measure the current. Anything over .05 amps (50 milliamp) is out of specification.

2) The Alternator is not charging the battery. Easiest way to find this is to measure battery voltage right at the battery with the engine off. You should see 12.4 or so volts, but anything from 11.8 to maybe 12.9 is fine. Now, start the engine and have someone rev it to 2000 rpm or so. Measure battery voltage - if the alternator is charging, you will see 13.5 to 14.4 volts.

Guessing at various items is a total waste of time. Use a meter and KNOW what is going on. DO NOT pay some fool who is standing there guessing about what is wrong, telling you about how his uncle found battery drain by replacing fuses or whatever - if he can't use a voltmeter and ammeter, it's time for you to go somewhere else.

Of all the electrical problems you can have with a car, a battery that won't stay charged is the easiest thing you could encounter.