In laser hair removal, light energy is transferred to key areas in the hair follicle. Wavelengths of light energy blow up cells and structures needed for hair growth in these locations. It is important to note hair growth is not permanently stopped by laser treatments; it is reduced. Thus, lasers are said to be used for hair reduction, not removal.
The key area for hair growth is called the hair bulge or bulb. It is fairly deep in the skin, making sufficient laser energy penetration to that level difficult. Lasers must generate enough power to blow up enough melanin in the hair bulge to alter hair growth. Laser light may be prevented from reaching deeply into this portion of follicle if it hits other targets higher up like melanin in the skin or hemoglobin in blood vessels. These can absorb the intended energy too.
It is important to understand that laser hair treatments are tricky. Not all lasers work well for hair reduction. Not all settings, practitioners, and skin types obtain good results. The bottom line is hair reduction laser treatments are actually difficult to do well. The fact that there are so many hair removal lasers and light machines on the market is a testament to this.