Underfill rework issue

Hi every one I would like to have some inputs here,I would be very appreciated if you can give your suggestion; Currently we are working on 2 underfilled BGAs with SRT-1800 rework station(hot air type),one is mounted on a single sided PCB whereas the other is mounted on a double sided PCB.PCBs are FR-4 epoxy/laminate and have some thickness of 0.5mm.Design for two BGAs are same with 0.5mm pitch and 0.32 pad size. It was strange that we reworked BGA mounted on single sided PCB sucessfully without any pad loss,however,failed in rework of double sided PCB with 1 pad loss at least. Can anyone help me out?Any comments would be greatly appreciated.Thank you in advance!

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