- If the transmitter is designed to work at 200mW then it may simply need some airflow around it. I'm guessing it operates at 5.8GHz?
- As installed maybe the antenna isn't radiating well and a poor match to the transmitters output FET - try placing the antenna outside and away from metal objects, battery and receiver. Similar problems occur if the antenna connectors are dirty, worn, damaged or not tightened.
- Maybe locate the antenna to the underneath of the glider and orientated to ensure the radiation pattern is downwards to your receiver. You may then get away with a 25mW setting? A hemispherical pattern antenna radiating downwards is probably the best option for a glider.
Just be aware, transmitting at 200mW in the 5.8GHz band in UK would require a licence, the limit for unlicensed at 5.8GHz is 25mW:
https://www.ofcom.org.uk/spectrum/rules/licence-exempt-radio-use/licence-exempt-devices/Radio-controlled-models
and scroll to the table at the bottom.