Very probably because they are! Before you can achieve good solid arrow flight there are a number o things that you need to do to insure that everything on your crossbow is properly tuned. Then you would need to go through an arrow tuning process as well before you can be sure what is causing this problem.
There are many articles posted on the earlier pages of this forum that talk about these steps. I would recommend you do a little reading to gain a basic understanding of how to set up and tune your crossbow. Then, if you've done these things there are number of members including myself that can be of help to you.
I would suggest to start with the Undocumented PSE TAC15/15i Information. It's long, but well worth the reading. Also Len has monthly feature articles for each month on this website. Try reading the feature articles for May, June and July on Arrow Spine and Tip Weight, Speed and Kinetic Energy, and Arrow Fletching and F.O.C. Balance. These articles are the key to understanding most of what you need to know about arrow performance and how to attain it.
Without this knowledge your questions are way to broad.