Gary, that sounds more like a mechanical issue. LS is not the cleanest powder in the world, but if it lights at all ... there should not be much/any unburned powder in the barrel.
Regardless of how they sounded, did the shot break the bird? the wad go past your shoelaces? etc.
Check for your primers may have gotten wet, or maybe sprayed with WD-40 or something. Shells may have been wet? Condensation? yanno, from inside an A/C house to you hot, humid garage and you filled them up right quick?
A 'fizzler' primer will cause what you described, but they don't much happen out-of-the-box ... and NEVER 6 in 25.
Another possible explanation is wet powder. Did you leave the lid off the jug, or leave it in your unplugged-up loader for a few weeks? Powder WILL absorb water, and it can be both sporadic and hard to light.
Bob