Digi tv problem?

Kenmark tv, Sony top box, Digital aerial external on chimney, facing Isle of Wight transmitter approx. 60 mls away.

Every time we get a nice sunny day some of the free serve stations come up with 'No signal' why?

Its a pain in the arse since they changed from analogue to digital when it effects receiving. Not every time but on re-tuning ended up with 10 French TV stations, would mind if I could understand French.
Many thanks