Problem: x5 won't start. What could be it be?

[Werewolf]

I could use some help.

I recently put my x5 on a battery tender b/c I wasn't driving it much recently.

Anyways, it is fully charged when I take it for a very short drive today. After an hour, I start it up again to drive it home.

I come back 4 hours later and the car won't start. When I attempt to crank the engine, I hear a click click click and my headlights (full, not daytime) are flashing on and off in time with the click click click.

I end up hooking the x5 up on jumpers to another running car and after a few attempts, it starts.

Any idea what the problem could be? My local dealer told me my battery was fine when I saw him 3 weeks ago.

Thanks in advance