Old firmware versions (pre-"Bastille", 5.12.88) only collect EPG data for the group of channels associated with the currently viewed channel (i.e. those on the same multiplex (or mux)). As you change channel, more data is collected but this is reliant on the broadcaster to actually be sending the correct data (and some muxes are occasionally quite lax about this — either wait for them to sort themselves out or upgrade your firmware!). This is why the Toppy's EPG on these firmware releases actually changes channel as you browse the Guide (if it didn't there would probably be no data to show for some channels). Another issue is that these old firmware versions can only hold about half (or less) of the complete list of EPG data currently being broadcast on Freeview (about 10,000 programmes!), so inevitably the data will be incomplete or have problems.
Newer firmware versions (5.12.88 onwards) collect the EPG data for all channels when tuned to any channel (assuming the broadcaster is sending it — the main BBC channels are pretty reliable). And they have room for the whole Freeview 7 day EPG, although there are still some bugs that cause duplication and waste EPG space.
The channel organisation on Freeview is sometimes shuffled about with no obvious change (the channel may still be viewable) apart from missing EPG data, or your local transmitter may be experiencing problems or maintenance.