It's either got an infestation or infection of some kind, or temperature fluctuation - I've no idea what your weather's like over there, but if you were in England, I'd suggest one of them is in a cooler micro climate...

Thanks,
They are very close to each other, so I think they are getting the same of everything. The temp right now is in the 70F or around 22 degrees, in the day time.

Not cold, then! Turn the one that's dodgy out of the pot, check there's nothing inhabiting the soil - over here it could be vine weevil, not sure what you get over there.