As noted, first make sure the shuttle goes all the way back. My LnL didn't come with a primer follower rod, so I made one of wooden dowel. I made mine long enough that it would definitely drop into the shuttle when the tube was empty. So to test if the shuttle is going far enough back, I just put the dowel in and see if it drops when the shuttle "should be" at its farthest back point.
I also put a weight on the dowel. I had a plastic drywall anchor handy, so I slipped it over the top end. Then I put a damaged 9mm case on the anchor - which held it in place on the dowel. Works like a champ.
Note that different brands of primers can have slightly different outside diameters, so they may flow differently through the tube. The first thing about this is that it helps to clean that tube now and then, to avoid gunk building up.
Here's a graphic (stolen from Accurate Shooter) that shows the differences in brands' rifle primer diameters:
View attachment 1092354
SAAMI specs for "small" primers call for a diameter of 0.1745" to 0.1765", while large pistol primer diameters can be between 0.2120" and 0.2122", and large rifle primer diameters can be from 0.2105" to 0.2130". You can see that the manufacturing variations between makers are "within specs," but can give you a different experience with each brand - and type.
Keeping the tube clean is useful no matter what, but with some brands of primers, a follower with a bit more weight helps a lot. And it's not just "largish" primers that need this help..."smallish" primers can tilt a little and bind up in the tube.
So, long winded answer, but it comes down to keeping the machine aligned, and "helping" gravity feed the primers.