signal strength for DVB receiver

I have a scart DVB box and I get about 40 channels, however signal strength on some channels fades and these cannot be viewed. Can somebody suggest ways of improving signal reception.

is this by aerial and might it have moved?

Signal is received by external aerial. Tried several positions of the aerial but problem still persists. This only applies to some channels (probably the weakest ones.)

some uk areas may differ as to who you get signal off,some dvb/t boxes had a setting for powering antennae.
indoor or outside aerial in use.

My box has the setting of powering the antenna but I did not notice any difference in reception. I also have a facility of choosing the country. The result is that you get a set of slightly different channels for a particular country.