Lee must offer loads that have a matching slot in a disc. They also must load to the smallest disc, so that a guy won't decide to load to the nest larger hole. They choose a powder and data from the maker, a nd then change that load so that it will work with the disc.
Why can the powder makers load genuine numbers instead of tweaking them to fit a disc? The presumption is that you own an infinitely adjustable measure such as a uniflow, not a measure that only works in present units.
This is why I believe you had failures. I am going to presume that your loads are lighter than accurate information. Five is a good powder for nine, but it does not generate identical pressures, and your velocity May be very low. I believe that your rounds are simply not generating enough recoil impulse and it is building up slowly, so you don't have a strong enough push.
As was said, try raising your charges, stack your discs if you can find a way to get the higher, yet acceptable charge. Don't do light crimps. It is a path to disaster if a bullet slides around in the case, and a powder charge must have a tightly contained area for complete ignition. If that crimp pops open before the charge is fully ignited, it may actually leave the barrel before that charge is fully ignited.
I'm going to say it. I suggest that you dump the disc and make the much larger expenditure for a genuine cast metal measure like a uniflow or any over the others. If you won't, doesn't Lee make an adjustable bar for that thing? Get the bar.
You seem to be a very conscientious and capable guy, and made pretty good decisions on your load, but as a load, assembled that way, it caused failures. If you were using a revolver, you'd have probably had no problems, but semiautomatic weapons Must have special care, or the round will not be capable of generating the energy needed to fully cycle the slide. A stovepipes confirmed that, it shows that the slide didn't have enough power to eject the brass.