Key fob locks but won't unlock

[Pingouin]

Hi all,
A new problem developed on my 3dr 2008 Yaris this morning: although the key fob can lock the doors, it won't unlock them. I tried the spare fob, and it did the same. After many attempts, it may successfully unlock once in a while. The inside button does work though, and I can still unlock the driver's door with the key. I thought it could be the fob battery, but then the lock button always work, and unlocks is really struggling to work even when using the fob inside the vehicle. Besides, the little red light on the fob works fine.
Any ideas what could be wrong? It's still under warranty, but since it seems to work randomly, I bet that if I go to the garage it will work, they'll tell me that they can't fix something that works and I'll be wasting my time.

*EDIT* it seems that if I point the remote from the front window towards the back of the vehicle, the likelihood of the fob successfully unlocking the doors increases significantly, so I may try to change the fob battery anyway, albeit as I said before locking is working fine.