You have to ask yourself, is:
1) the battery not being charged up (as you ask in the subject line)?
2) the battery charging but being drained by an excessive leak or short over the two days you indicated?
3) the battery not able to hold a charge over the two days?

To test (1), check the battery's charge as you start up. If cold, it should initially be charged up to about 14.4 volts (approximately), then gradually diminish to about 13.8 volts and stay there. If this is what is happening, then your charging system (alternator, voltage regulator) is good. If it's charging up to 14.4, and not diminishing, it could be a sign your battery is defective -- how old is it?

To test (2), follow advice of Grifman and my followup to his suggestion.

To test (3), isolate the battery by disconnecting the negative cable (not the positive) over the two nights you leave it idle, and then see if it can start. If it can't start, your battery has lost the ability to hold a charge, a sure sign of old age (sulfation) -- replace it and all will be well.

Good luck.