I recently edited Colette Marence to be a Master trainer instead of an Expert one, however the change did not 'take'. I tested with a character that had never been to see her before, as well as one that already has, and in both cases it thinks she's still Expert-tier. I tried changing her TrainerType value from 2 to 3, as suggested in a topic about such edits in general, to no avail, and also tried disabling and then re-enabling the mod in question.

About the only thing I can think of at this point i that I overran some hidden limit to the allowed number of Master trainers for a given skill (it says I have three, including the guy from Dawnguard), however I couldn't find anything related thereto in the Settings list. I have not yet tried downgrading one of the other two, lest I screw something else up in the process, however I suspect that it wouldn't matter.

There should be no built in limitation on the number of trainers at a certain level. Otherwise mods that add trainers to the game would not work.

One thing to test is to start a new character and see if the trainer is doing what you want.

Also, make sure the mod you are making loads after any other mods that change anything about Trainers in the game, or use BASH to make a Bashed Patch to merge the lists together if for some reason it is loading earlier. Speaking of which, for testing purposes, make sure the mod loads after the Bashed Patch so it's leveled lists do not overwrite the ones in the mod.

As for the particular NPC, did you also change their skill level in what they train? That could have an affect on how the game display's their training level, if not the level that they can train to.

If you mean her inherent skill level, then no, I didn't change it. It's also the only mod I have that alters trainers, so that's not an issue. I'll try a new character and see what happens, the one who hadn't visited her was made before the change.

EDIT: just tested with a character newly run through Helgen, and she's stilll stuck on 'Expert' for some reason.