Go through your exact sequence of keys for a std fight.

Do you have /follow and /assist bound to the same key (ie. you hit #1 and the macro bound to the #1 key is set to have the alts /follow & /assist)?

Here is what I have setup. On my alts I have the movement keys "w.a.s.d" unbound. Then I made a /follow macro and bound it to the "w" key. That way everytime I use "w" on the main to walk forward the alts are being told to /follow all the time. (this works great for me but isnt ideal in some situations, you may not always want your alts following)

Then across my numbered keys I have the following macros /assist "main toon" /cast "spell" - that way when I want to cast a fireball for instance I hit the #1 key on the main and it then causes the 2 alts to assist the main and start casting a fireball. Same goes for the other #'s I have the assist macro and cast macro on each for whatever type spell I want them all to cast together.

Now this is a very simplistic/rudimentary way of doing it but it also is what works for me.