Trophy Rds Radio Reluctant To Retune

The radio on my 2012 Trophy SE is pretty good on the whole. I use it with a Sena SMH10.
The problem I have is that it is reluctant to retune to the local FM frequency for national stations using RDS, hence I start out with a good signal, which then degrades as I get near the limits of its geographical coverage. Sometimes I can manually retune to the local frequency, but this is not always successful. By and large i am listening to BBC Radio 4 which has broadcast frequencies around 93 - 94 mhz.
Any ideas??