Pods that don't fill out might not have been pollinated.

In many parts of Georgia it is just to hot and dry for even limas to set. That is the case here. If it cools a bit and we get a rainy spell, they will come on. Other problems include low pH. limas like a neutral soil, plants in acid soil will experience more blossom drop. Some varieties are also more prone than others. Most reliable under adverse conditions is the Jackson Wonder.
Of course if you are talking about the edamame varietie "Butterbeans" than that is a different story.