heating problem

hi we have a 90,000 btu boiler that heats 13 radiators, 9 of these are on a 3/4" pipe and heat ok, problem is the other 4 are running off a 1/2" pipe (put in by a professional plumber) which now runs under out kitchen floor. These radiators do not heat as well as the rest of them, what is the max number of rads for a 1/2" pipe?. Will getting a higher output boiler solve our problem??