Sounds like (if it was a bunch of neutral guys) u've got your highest-lvl guy on guns ,
leaving someone unqualified to be BO on the ship. You should be able to use your
National BO on a neutral ship & leave your neutral guys for guns, if you've already
nationed them.

Just check the lvl of the ship & make sure your bo is of adequate ability...

It worked before and i have nuetral ship but last won't go in. and my royal ship
where i trained new guy in my support crew where i can put 3 and i did put before.

You checked to make sure that adding it won't put you over your max displacement?
As crews get bigger, you can often be caught out....

Alternatively, if there is equipment on that spot higher than his lvl permits, or of a
nationality if he is neut......that won't work either