Flarm effectiveness utterly depends on antenna configuration. Things like engines, fuel tanks, metal, carbon fiber and human bodies block transmission. Fiberglass and fabric are mostly transparent to Flarm and radar frequencies.
External antennas on top and bottom can work well on powered aircraft. Many gliders have Kevlar fins and some manufacturers offer an opinion to place antennas there with the necessary coax. You would also want an antenna in the nose. Or antennas can be placed inside the canopy or windows on each side.