How can you make a light source brighter?
You have a few choices. Buy better LEDs and run them at the same current, run more current (assuming that's possible while remaining within ratings) through the same LEDs, or some combination of that and adding more LEDs.

More directional lenses or reflectors may concentrate the light there is into a smaller angle or area, but there is no practical way of getting more total light power from an LED than I mentioned in the above paragraph. Diffused LED packages emit light in all directions so they will have the lowest luminous intensity, all other things being equal. The same die, at the same current and other conditions, in a package that emits with a narrow angle will appear much brighter.