I was quite certain that if you leave it at "10 minimum" they just all go to work on it despite not having 10 squad members.
The defining a barracks for them thing should be what "fixed" the issue for him.
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Now I should mention that there are a lot of ways you can slow up the already not-fast skill gain from training. If you actually tell a 10 dwarf squad to train they are unlikely to become any good for quite awhile. When a dwarf learns something they will often do a demonstration to teach it to the other dwarves but they will wait for everyone to actually show up, even if they are eating or whatnot. Add on that demonstrations do not give a particularly large amount of experience when the teacher is in the first couple of levels and you might be so unlucky as to have almost all dabbling skills on your recruits for entire years in the game.
Instead make squads of 3 with training orders set to 2 minimum. They spend a good split of time between actual sparring and demonstrations like this and you won't have them waiting on the slowest guy that could get anything from some demonstration nearly so much.
Moreover though, when you do actually have a squad of master-to-legendary fighters you should split them up and place them in charge of their own squads. They will actually teach their subordinates quickly once they are good.
*skill gains seem to go mainly fighter tailed by their equipped weapon, followed by shields. Don't worry too much about armor and dodge- fighter actually has a big role in if they get hit or not so they can level those skills in live combat or very long term training.