3rd retrival and disappointed in numbers

We had our 3rd Retrival Saturday and got 9… only 9 and from the ultrasound it should have been close to 20– I don’t know or understand what happened. Our update phone call today was even more disappointing— 6 mature but only 3 fertilized—this is the lowest numbers we have had and I know it only takes one. But we are also doing PGTA testing and I just really worried we will t even have one to transfer — wondering if we should do another retrival back to back ?… I don’t know