Sometimes the fault lies with the generator and voltage regulator. There is some truth here: malfunctions of these devices are noticeable by the "movement" of the light flux, as if subject to the number of engine revolutions. It is clear that poor operation of the generator and voltage regulator affects not only the efficiency of the headlights - first of all, the battery suffers, either not receiving a full charge, or, on the contrary, receiving it in excess, which causes the electrolyte to "boil". In this case, the headlights only play the role of an indicator, although excess voltage greatly reduces the life of the lamps. Constant dim radiation occurs for other reasons: either the reflector becomes heavily contaminated or corroded, or a dark coating appears on the inner surface of the lamp bulb.
If the durability of the reflector depends on many factors, then even a completely serviceable lamp "accumulates" a dark coating in some 30-50 thousand kilometers. In this case, its efficiency is 20 percent lower than that of a new one.
The reflector "lives" longer. In modern cars, the reflector and the headlight glass are, so to speak, an indivisible whole, which first of all ensures the most optimal light beam, and secondly - tightness. It was the lack of tightness that allowed dust to penetrate into the headlight, which then burned onto the reflector and reduced its light output. The same thing happened with moisture, which actively condensed on the reflector, causing corrosion.
About five years ago, they tried to combat corrosion in some incredible way, pouring red brake fluid into the headlight. The fashion for this idea, which was more harmful than useful, has passed, but another extreme has appeared - plexiglass "glasses" on glass. In addition to the fact that such protection reduces light output by "smearing" the direction of the rays, it causes strong heating and burning of the reflector. What good is stone-protected glass if the reflector suffers, is unclear, especially since, as has already been said, the glass and reflector are now an indivisible whole? By the way, that is why there is no point in buying a "lens" separately: you will never get good light from such a headlight.
Finally, regarding the adjustments. Even the best headlights equipped with halogen lamps may be useless on the road if they are adjusted poorly. Not everyone has the opportunity to adjust them on a stand, and not always. Therefore, old, well-tested methods of "home" adjustment give quite good results. The car is placed on a level surface, 30 meters are measured and a brick is placed on the mark, for example. One of the headlights, turned on low beam, is covered with something, and in the second, the adjusting screws are turned so that the illumination boundary is brought vertically to the placed brick. The same is done with the other headlight. The high beam is adjusted horizontally, but without a brick: here it is important to ensure that the beams from both headlights do not split into two and do not converge in the middle.