solder question on crystal in DFN style package

We are installing via reflow soldering a crystal that has 2 pads, 1 each end. They are flat pads on the bottom of the part with a small area of the pad looping around and up the side of the crystal on the end. Inspection of the cyrstal under a 40X microscope after reflow show a good solder joint with a proper fillet on the end where the pad is exposed. However, our EMS customer says a few do not work until they reflow the joints on both ends of the crystal with a soldering iron tip.

Does anyone have experience with this type of package? I have attached the data sheet showing the crystal and recommended pcb pad layout which is being used. Any ideas as to why some of them don't work unitl they are reflowed again with a iron? All other solder joints on the board look (and electrically test) good so I believe our reflow profile is correct for this board. We are using lead free solder.

If the attachment link does not work, here is the address to the data sheet: http://www.citizencrystal.com/images/pdf/m-cs10.pdf

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