Hi Mally
The Cal. 3120 inside Royal Oak 15400 has 60 hours of power reserve when fully wound. From what you just described, it seems there is nothing wrong with the watch since the watch is working properly when you manually wind it. From my past experience, there could be couple reasons:
1. It's been 3~4 years since your last service, depending on how often you wear your watch, the oil may have dried up already. The watch just needs to be cleaned and lubricated again.
2. Since you are on vacation, there might be a chance that your arm just isn't moving as frequently as when you are in the office working. If your arm isn't moving, then the rotor isn't winding.