Could also be shooting directly over it vs a little offset. I'll play around.
I have found my Garmin to be a bit finicky until you've used it enough to understand the set up and what delivers consistent repeatable results.
My Pro shop set theirs up for me and insisted that you should not shoot over it!! (LOL). I told them that they were wrong (they were) but they didn't want to listen to me. Went home, did some research, confirmed that I was correct. I came back two weeks later and set up my Garmin Chronograph and their's in series to do a little testing...of course shooting "5 inches to 15 inches" over the tops of them.
IIRC, in order to obtain reliable readings, it is important that the target is ten yards beyond the chronograph. Or something along those lines.... Also, in my usage so far, the devices seems sensitive to the distance from the launching point. Meaning, based on a few sessions it "seems" that I'm more likely to get funky readings if the tip of my arrow is , say, only 36 inches from the device. Since my arrows are 29.5 inches long, with a 36 inch distance they should have been released from the string and yet I get funky readings. The last time I used the unit I stood about 6 feet from the device and obtained consistent, repeatable results through a series of about 30 shots (testing different arrow weights).
I haven't looked very hard for Garmin trouble shooting info to address proper set up and inconsistent readings, but I don't think it is addressed in their, um, "instructions".
No doubt there are about 20 worthless YouTube videos on the Garmin and maybe 2-3 really good ones. I haven't taken the time (patience) to sort out the good ones from the lousy ones yet. Generically, DC Rainmaker does the best Garmin reviews - nobody is even close. In fact, I've never seen anyone do a better job reviewing any sporting technology than DC Rainmaker. Even Garmin refers people to DC. But he is a cyclist / triathlete so he may not be a good source for info on the Chronograph.
When I'm shooting one arrow three times with proper set up, I usually get two virtually identical readings and one reading not more than 1.0 to 1.5 fps different. I have not yet shot a long string of identical arrows (or a series of same weight arrows). I have been addicted to Garmin products for, hmmm, probably at least 25 years. I only own Garmin...watch, bike computers, car gps, RINO radios, on and on. I think I'm on their "VIP" list. I've always said and always told Garmin - they have the best hardware and the worst user interface. To that point, I feel that their instructions (and menu) for the chronograph are, shall we say "sub-optimal". But Garmin can't make anything without having sub-optimal user interface.
For fun, I'll call out two things on the Garmin Chronograph user interface. As far as I can tell, it won't let you proceed with a session unless you clumsily push the top buttons to remind it that you want "bow" mode, and you must click again to decline to input the arrow weight. Why not have it remember the last "mode"? Something isn't right with the input requirements, but wait..it's a Garmin. LOL. Now, if you do include arrow weight it won't deliver momentum - only KE. Personally, for my uses, I don't give a hoot about KE. If I want a number other than velocity, I want Momentum. If someone is developing a chronograph for shooting sports, and decides to ask the user for projectile weight in order to calculate KE, how hard could it be to include a few lines of code to deliver Momentum instead of or in addition to KE? Maybe it's there and hidden in the user interface....
Lucky for them, their technology is so superior they can still grow their market. Net, net I still love Garmin, but hate their user interface (whether it be instructions, troubleshooting, menus, you name it...). They also have awesome customer service.