Allow me to add emphasis to what Jmorris said about the plastic bushing. I had one fall out, unnoticed, one time, and not only got near squibs but near double-charges, as if the powder for one load were hanging up on top of the drop tube, then getting added to the powder charge for the next round. I should add that I was using a drop tube/operating rod that I'd made myself to a Lyman M type profile and that had a flatter top portion, so I don't know this will happen with a factory rod. But irregular powder charges in some degree can happen with one and never a positive.
When I found the errant bushing on the floor and figured out what it was and put it back, the problems stopped. The symptom during operation, other than the irregular charges, was rocking by the measure during operation. If you ever see that, stop and look.