I tend to agree with Jez Smith. I wonder if the grounding has any effect on the crystal oscillating. But who knows, maybe working when lying flat is exciting ( for the crystal ).

But as a rule in all PCBs for MCUs, we fix the crystal flat on the PCB and have a wire that binds the metal case of the crystal to the ground plane by soldering. Obviously this is a more stable configuration as the crystals that we use, are quite "tall" next only to the electrolytic capacitors. Using them in the flat position helps to lower the overall height of the PCB, particularly when there are no electrolytics.



Raghu