Been thinking about it all day, here's my thoughts. I haven't been at it long, but I've done a fair amount of arrows and they've all come out beautifully.
I wonder (like you said) I it could have a bit to do with the tape. All of my arrows have been fletched with fletch-tite or similar glues and they end up with a nice spiral in the end. Seems like the tape may not hold the quill so rigidly in the initial setup. If I were you I'd try fletching one with glue and see if it gives the result you're looking for.
It also reminded me of what rw feathers would be like in a lw jig..... But I blew that off because it sounds like you've been at this a while and wouldn't make that mistake
my 2 cents, Austin