since boost is a measure of resistance to flow, if you your setup is very restrictive, the higher pressure may demonstrate that the engine is incapable of ingesting a larger volume of air.
the efficiency of your compressor is also important, if you are pushing air efficiently at 10 psi, and inefficiently at 14 psi, the air temp will be greater at 14 psi and therefore the air density will be lower, which could be resulting in a comparable mass of air entering the combustion chamber despite the higher pressure.
even if the compressor efficiency is comparable at 10 and 14 psi, the larger volume of air moving through the piping/intake/etc will itself generate more heat thereby reducing it's own air density, possibly resulting in the consumption of no more air mass despite the increase in pressure.
it is also possible that the engine is extremely tired and in need of a rebuild and that it's combustion chambers are unable to contain the additional mass of air that might be forced in at 14 psi compared to 10 psi. this would result in excessive crank-case pressurization, which in turn could blow oil out of various openings.
In short, any of the above factors, any combination of the above factors, and any number of other possible explanations make it perfectly feasible that you would indeed see no increase in power despite the increase in boost pressure.