Bribed garrisons are useable the same year, bribed field armies which are still flashing between colours aren't. The year afterwards, they often get "stuck", so that although they SHOULD be useable, you can't select them. There are two solutions - wait a year (which may not be an option...) or merge stacks (which has its own problems - maybe you don't want the units to merge.... but to get around this, of the units that you can control but don't want to be merged, either shift them across a border, or put them in the fort before you hit "M", then they'll be exempted from the merge )

It is a bug, and not amenable to modding.