Most broadhead flight problems are related to improper arrow spine (usually too weak), and many hunters aren't aware of it. There is nothing to indicate that you can still be spined incorrectly even if you followed the "shaft selection procedure" provided by the arrow manufacturer.

Fletchings with a lot of drag (feathers, for instance) can "mask" problems. You might get good flight (like you did last year), but you're actually treating the symptom rather than the problem.

The best way to check things out is bare shaft testing. Using field points only, shoot some fletched arrows and a few unfletched at the same target from about 10 yards. They should impact almost identically. If you can get them together out to about 20 yards or so, you'll be able to swap over to broadheads with little headache. Many people find that the bare shafts fly all over the place, meaning (more often than not) that they have a poorly spined arrow. You can make the arrow stiffer by dropping point weight (I wouldn't do that since you're already at 100), cutting them shorter (not always possible, cheap, and how much should you cut?), or by turning down the bow's weight.

There's one bit of misinformation in c903's post. Vanes DO add weight and can change the FOC (which the importance of is up for debate), but adding weight to the rear INCREASES the arrow spine (makes it act stiffer). Conversely, adding weight to the front decreases arrow spine, or makes it act weaker.