Forget the flip-over (round) pins you are using. They do not hold up in this application. You are using the cheap 10-29 cent versions. The more expensive 79-100 cent versions, typically black, have a much stronger snap action & might work better. Still don't like them.
Use the hairpin style. If you get a normal one, kick it on _all_ the way, not just the first notch where it normally should stop. But these still aren't the best.
There are hairpin style that have a twist lock on them. You have to spread it open, put it on the pin, and then twist it & the free end of the hairpin clicks over the straight end. These stay on. I'm in the middle of corn planting, but I can click a pic one of these days if you remind me.
In any event, I have only lost about 2 hitch pins in my 35 years of active farming, and rarely use a locking clip of any kind. Manure spreaders do like to get heavy in the back as they get near empty & pick up the hitch, then as you drive it chatters up & down on bumpy fields & works the pin out.
I replace those &%%^^ locking ring clips (linch pins?) with really strong cotter keys on my tractors' 3pt arms. Cornstalks always knock those silly rings off. Worthless.
--->Paul