Lot's of new parts and not enough checking
If the torque converter was not fully engaged or lined up with the transmission oil pump prior to install in the car, the converter may have been stacked up on the pump and the flexplate may have been FLEXED with the converter forced against the flex plate due to the hub not engaged properly in the trans. If you have a new converter and have not checked the air gap measurement between the flex plate and the converter prior to bolt up of the converter to flexplate, you could have either a too large a dimension on the hub to mount lug dimension, which will push the converter in to the flexplate and cause trans failure and thrust bearing failure in the engine. This dimension is expanded the thickness of the mid plate when a mid plate is used to maintain the proper 3/16" to 1/8" air gap between the converter lugs and the flexplate prior to bolt up and after the trans is tight to the bell housing. Washers between the flexplate and lugs will not correct the improper hub / pump engagement. If the gap is too great between the two when checked, the converter must be cut apart and the dimension adjusted properly. The crank pilot must be fully engaged to allow the assembly to spin true. When you use washers between the flexplate and the converter you are taking away from the .20" or less engagement that the converter pilot usually has when dimensioned properly. This is the likely problem if the car has a mid plate and the converter is built for a non midplate application.
There is about a 2% chance or less that you could have a bad flex plate but if it is new that is highly unlikely, very highly unlikely.
Bruce