A lot of people board Oasis and Allure and are conditioned from prior cruises to think of only the Windjammer for the buffet. So they go there and think 'this is so small for so many passengers'. The idea Royal Caribbean had when designing the ships is that people would avail themselves of the other places and not all rush to the Windjammer.
But you have to look around because the dining options are spread all over the ship. First morning at sea aboard Allure I wandered through the Windjammer and it was packed with people standing around looking for seats. I went forward to the Solarium buffet and it was about half full.
Use the touchscreen displays by the elevators. You can pull up real-time information for each dining location and see which ones are crowded and which have space open.