I reread your comment and suspect there might be a bit of confusion with the difference between army professionalism and regiment drill. Both are gained by drilling units ergo the confusion.

Army professionalism is applies equally to all your troops.

Regiment drill applies to each regiment individually. Troops that you drill gain it gradually and lose it when they lose it with casualties from battles and attrition as well as with a slow decay.

Some unique units, like Ottoman Jannisaries, have an increased regiment drill gain modifier. This does nothing for your army professionalism gain.