dd always fails to write one partition on first try; succeeds on second

For a robotics project, I've assembled a make target that uses dd (actually dcfldd) to write a Raspbian Jessie image to SD card. This way, I can be assured that my environment is reproducible from scratch any time I want it.

The make target is just

flash:
    sudo dcfldd bs=4M if=$(IMGPATH) of=$(SDX)
    sync

where $(SDX) is /dev/sdc, and is meant to be preceded by a script which mounts the image at /mnt/img, makes some modifications, calls sync, and than unmounts it.

The process seems to be working fine, except that I always need to call the make target twice--the first time, if I pull the eject the card reader and reinsert it, one of the two partitions from the image fails to mount, and, if I try to boot the Raspberry Pi from it, I get a keel panic.

What's going on here? Is there some other way I should be writing this image so that it works on the first try? I don't want to be buing my limited SD card write cycles needlessly.

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