It happens sometimes. Most times my arrows spin into a blur all the way in. Once in a while it looks like a knuckleball pitch where you can watch the individual feathers making maybe one rotation in 20yd. Next time I shoot that arrow it spins just fine.
And yes, I know how to fletch, use the same jig, same feathers, know how to shoot, etc, etc. Maybe there was a brief tail wind or something?????
I've seen it with field points as well as broadheads. They all fly great so I'm not too concerned with an occasional slow-spinner.