Have you tried swapping the two antennas? If the bad performance moves, you have a bad antenna or a bad mount. I have seen two antenna failures in my club; one a club glider, one a privately-owned glider. If you have an OGN receiver nearby, you can use "Glidertracker" to see your received power at the antennna (this is how I diagnose bad FLARMs, and also use a known good antenna to test the antennas); (click on your glider icon, a window pops up showing info including received signal strength at the receiver. In each failure, the power at our OGN station was 20 db lower on the bad antennas (OEM), about 50m away). I have saved shipping two FLARMs for unnecessary diagnosis by discovering bad antennas. Antennas are cheap, and they sometimes arrive dead, or break, particularly if they are often touched/handled/'twanged'. Link for Glidertracker is https://glidertracker.org/#lat=1105585&lon=6428048&z=5 . You can zoom it to your area. OGN stations are inexpensive and very useful for crew wondering how their pilot is doing.