right...same here....
in 80% of all tries, it is not possible to merge, especially when (but not limited to) they have a complete roman name (with proper cognomen etc.)

only solution is, to move the admirals ships to harbour, build a new one and then transfer one of the ships to another fleet.
to admiral takes always the best ship (because the RTW engine moves better ships to the first rank, e.g. quadrireme over trireme)....so you can also move ships which were the admirals ship before.