There is a possibility that the FLARM, if fitted, was in 'Stealth mode', which is used in competitions.
I put my FLARM Mouse in Stealth Mode and OGN-R cannot see it at any range. Tried to get my head around how this mode works (or should). I thought it was the receiver that was blocking the data, therefore OGN-R should be able to recieve what was transmitted.
Since I have Thame OGN-R back up and running now, I could try some more tests.
3.1.7 PRIV
Description:
Stealth mode. Instructs all receiving FLARM devices that the received data must not be made accessible in real-time full precision, except for the purpose of collision warning. Real-time is understood to be within less than 10 minutes. With the option set, not all information is forwarded to connected displays to prevent abuse in competitions. An activated stealth mode results in PFLAA sentences not carrying specific climb and cruise data while some random noise is applied to altitude data. It is recommended NOT to activate stealth mode! To apply full reciprocity, a pilot who enables stealth mode will only get information as if all other aircraft had enabled stealth mode, independent of their actual setting.
Also found this report from 2015 on Stealth Mode trials.
https://members.gliding.co.uk/2015/07/07/competition-rule-book-amendment-flarm-stealth-mode/And this page may also be relevent
http://wiki.glidernet.org/opt-in-opt-out