Never had that problem with my PEC. Make sure the lower link arms are tight up against the implement (dead blow mallet or a good kick), then slide the washer, bent flange on top against the lower link "eyeball" and insert the lynch pin (flat side of the pin against the washer).
I think Greg_G has the mostly likely answer. If it is the pins, I think Tractor Supply has them.
What I have found was the spring locking hoop of the lynch pins interfered with the washer's flange on occasion. If that happened, the hoop didn't lock down against the pin.
This appeared to be due the variation in the bend of the washer. Didn't happen with all the washers. A bit of grinding on the flange solved that problem.
Another issue I had now and then was the spring hoop on the lynch pin would pop up and I'd lose both the pin and the washer. I attribute this to the spring locking hoop not being strong enough to lock in place and movement of the implement (RFM mostly) in use.
This problem was solved by using a lynch pin with a stronger spring hoop. They are black and Tractor Supply sells them. You'll know you got the right ones if you can hardly get the spring opened (I frequently had to use a flat blade screwdriver for more leverage).
DO NOT let them spring closed on your fingers!!! If you've ever had a mouse or rat trap close on your fingers...it's about like that.
So, in conclusion:
1) Make sure the spring hoop closes completely (grind the washer flange if necessary)
2) Use the stronger spring hooped lynch pins
And one more: Order a half dozen or so extra washers from Pat's. Might not need 'em, but you're screwed if you don't have one when you need it.