A lot of show breeders say trimming permanently destroys the look of the coat -- makes it go very curly, some say dry and coarse as well. It does take *forever* to grow back in on most dogs. So most likely you aren't seeing that it is permanently short but how very long it is going to take to regrow -- could be well over a year if let entirely alone. I have a very heavy coated female whose feet were trimmed by a groomer even though I had said not to touch them
and a year later they still are not even a third the length they once were, and she was probably only a year old at the time
. They also thinned and trimmed some of her coat and that has never come back in the way it was initially but she was so badly cared for (she's a rescue and was in terrible shape) that it had to be neatened up to some extent.
Additionally, coats vary widely. I have two dogs with very heavy coats and two with very lightweight coats. The light-coated dogs are twice as old as my rescue with the heavy coat and her coat is much longer than theirs ever has been or probably ever will be and was when she was only one. The light coated dogs have quite tight coats on their backs and sides; all the length is in the feathering.
To me it sounds like you probably have a lighter-coated dog anyway and one whose coat grows very slowly. I would really consider whether you need to trim the coat -- this isn't a breed that is supposed to be trimmed at all (groomers who don't know this incorrectly trim them like cockers all the time!). If people want to trim I suggest a
knowledgeable groomer familiar with how this breed is supposed to be groomed (seems that 95% haven't got a clue in their heads about cavaliers which is shocking for professionals and is one of my real grumpy points!) who can neaten up the hind legs if you have a dog whose feathering gets pee on it regularly, likewise belly hair in the same situation, trim feet if they are an issue, and maybe, if the coat is heavy, thin it -- not cut it! Some take length off of ears but I think the longer and fuller the better -- IMHO the ears and tail are the glory of the breed!
Once it is cut, you have probably permanently changed the coat though at least in consistency, going by what experienced show breeders say.