Mirrored BGA Assembly

We have massive void problems on one of the BGA populated on the bottom side of the board. The board have 6 BGAs on the top and 1 on the bottom. No voids were observed during the first reflow (we reflowed bottom side first), but after the second reflow i.e.to reflow the Top side, voids were observed on the bottom Side BGA (which is designed mirrored to one of the top Side BGA). Being new on this type of assembly, I would appreciate if any of you experts can give me some thought to this.

regards,

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