What area are you?
If you are on the crystal palace group of transmitters there will be work going on throughout the day as each transmitter in the group gets switched at different times with some of them not being switched until mid afternoon, so I'd suggest trying this evening.
Also the switch is done in two stages, stage one is today, stage two is on the 18th, so until stage two is complete you may not have all channels. If you've still not got all channels after the 18th you may need to upgrade your antenna.