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I'm not having any problems with it, recently or in the past. Maybe be more specific with the issue that you are having. It's not the most intuitive UI, but it works if you know how to use it.
Usually a combined fleet consists of several different types of ships, to combine this in the game more easily, gather all your ships in one fleet and send them to the same port. Then go to the option create task force, then create the task force the way you want your combined fleet to be, then separate the task force from the main fleet and your combined fleet will be formed.
Just in case this is relevant, the only issues I have had with merging fleet or assigning ships to fleet was when one of the fleets was in combat. Could this be the issue which is bugging you?