Channel five is notorious inasmuch as it seems to receive a weak signal. If my connection is loose in any way whereas I can still get a good signal on all the other channels Five is always either pixelated or no signal try checking your connections.
That's your best bet. Don't know where you are in the country, but here in Cardiff in certain weather conditions I can scan in the Wenvoe transmitter (which is the right one) And the Mendip repeater (lousy signal)
A complete re-scan should sort things out, if not, you may need to consider a DBT booster (about £8 from Tesco's)
If you are getting signals from two transmitters, go through your channel list after scanning and manually delete the weak (repeated) ones.