It's sad that the Redwall series has only gotten two direct sequel books---we frequently have to keep adjusting to new casts of characters, although during the later books it got particularly bad, as Jacques didn't seem to want to bring anybody back. Still, in the sequels, in which the same cast did return, Jacques still failed to bring back the entire cast.
Mattimeo managed to include most of the cast of Redwall that survived, although there were two very glaring omissions---Dunwing and Mr. Squirrel. In the final chapter, the former is confirmed to have survived, and the latter is implied to have. Yet neither is even so much as mentioned---it's as if Jacques forgot about both. Whereas someone pointed out that sparrows might age quicker than other animals, this doesn't seem to explain what happened to Mr. Squirrel, since his wife, Jess Squirrel, is still very much alive, and there isn't any indication that she was a lot younger than him. Mr. Jacques himself was asked about this, to which he replied, "No particular story is attached to Mr. Squirrel." Your guess is as good as mine as to what he meant by that, seeing as Mattimeo is clearly a story attached to Redwall. My best guess about what really happened to him is that Mr. Jacques forgot he existed, seeing as he was nothing more than a prop to the story to begin with.
Next---and last, I guess---we have The Bellmaker. This one suffered worse, I'd say. Several characters, such as Friar Alder and Brother Hubert were missing in action. There is a brief passage mentioning that several Abbey members have "moved on to quiet pastures", which could imply some of these characters died of old age; Mr. Jacques likely did this since he felt they had no use to the story, even though they were undoubtedly a part of the cast of Mariel of Redwall. The most glaring ommisions, though, are that of Bagg, Runn, and [[Grubb], who were dibbuns in Mariel of Redwall, were alive and well during the prologue of Martin the Warrior, and whom Jacques seemed to really enjoy including. Obviously, their failure to appear was inexcusable. Sure, they wouldn't have done much to the plot, but the fact that Jacques bothered to put them in the beginning of Martin the Warrior makes it just shocking that they didn't appear in a book whose whole story included that cast.
One more thing...on the character's page, someone pointed out the ommision of the robin Chibb's failure to appear in The Legend of Luke, as many other characters of the book he was in (Mossflower) do. And many other, I would like to point out that many others don't as well.
I think it's important to notice these disapearences and discuss them. Because it's frustrating enough how Jacques changes his cast, it's even moreso that he won't keep a whole cast of characters.