How did you find out that one of the spacers was missing? Was this through working with the PSE Product Support engineer?
I found the problem after evaluating the limbs/cams after the string jumped off. I noticed I could slide the right cam up and down on the pin just a bit. I mentioned it to PSE and he said that those plastic spacers to sometimes break after a string derailment.
I used a washer to make up the new shim.
I just got the x-bow re-string and cams aligned, but ran out of time to fire. So I will test tomorrow.