If you want a more ‘techy’ article on the subject:
Of course, the Navy still wants its laser to target and destroy incoming threats, and therein lies the challenge. The lower power threshold for a weaponized laser of this nature is more or less 100 kilowatts; the FEL at the DOE lab where the Navy has been sponsoring research currently runs at about 14 kilowatts. Boeing’s job is to make up the difference so the Navy’s FEL can perform tasks that require 50 watts, 100-plus kilowatts, and everything in between.
Sounds like it's a problem of scaling it up.