Occasionally the way the Freeview channels are organised is shuffled about a bit. Sometimes new channels are added, some stop broadcasting, there are name changes, or things are just moved about for technical reasons. This shuffling can sometimes confuse the Toppy (and other Freeview receivers) with symptoms such as missing EPG data or the wrong channel being displayed.
To remedy this you will need to do a rescan to get the correct channel list. If just a few channels are affected then you may be able to simply delete the old channels and scan for the new ones, but it's usually safest to do a Factory Reset (which will also delete all your current timers) and then a complete rescan.
If you are running an older firmware (pre-"Bastille", 5.12.88) then the EPG data for some channels may be missing because the specific broadcaster is having a bad day and is not sending it (or transmitter problems or maintenance). This is much less likely to be a problem on newer firmwares which can collect all the EPG data from any single channel.