One reason is the difficulty of seeing always where the weak spot is; but a greater difficulty is due to the fact that the principle as above stated must be modified by the consideration that things which are important need attention more than things that are unimportant.

A weak point in any organism deserves attention more than a strong point of the same order of importance, or than a strong point in the same class; but not, necessarily more than a strong point of a higher order of importance, or a strong point in another class.

It may be more beneficial, for instance, to drill an ineffective turret crew than to try to reduce friction in a training gear already nearly frictionless; or it may be more beneficial to overcome the faults of a mediocre gun-pointer than to develop still more highly the skill already great of another gun-pointer; but, on the other hand, it may be less beneficial to drill boat crews at boat-sailing, even if they need it, than to drill them at landing as armed forces on the beach, though they may do that pretty well; or it may be better not to have boat drill at all and to get under way for fleet drill, even though the ships are very expert at it.
It is true that in any endeavor where many things are to be done, as in a navy, it is important that nothing be neglected; and yet, under the superintendence of any one, there are some things the doing of which requires priority over other things.

The allotting of the scientifically correct amount of time, energy, and attention to each of the various things claiming one's attention is one of the most difficult, and yet one of the most important problems before any man.

It requires an accurate sense of proportion.
Naturally the problem increases in complexity and importance the higher the position, and the greater the number of elements involved--being more difficult and important for instance in the office of the commander-in-chief of a fleet, whose time and attention have to be divided among multitudinous matters, than in that of captain of a single ship.