So all my dens are protected by master assassins but i'm not sure how to get my level 10 assassins to progress any futher... what am i missing?

Go to one of your new Assassin Dens and assign them at the table. Then you do a mission or two with 'em and they rank up.

I'm having the same issue too. Once I get my seven dens locked I can't level any other assassins past 10. Anyone know a way?

You can only have 7 master assassins.

The others will never go past 10, unless one of your masters dies on a mission.

also even if they're stuck at level 10 and they've been going on missions when you finally assign them a den and do their first mission the accumulative points will come back and rank the assasin up as originally intended.