The sensitivity of mammography is low in young females because :
The mammographic appearance of breast depends on the relative amount of fat and glandular tissue which are present. Young woman's breast contains a large proportion of glandular tissue which appears as a soft tissue density and lowers the sensitivity of mammogram.
Mammographic appearance of breast (Based on the relative amount of fat and glandular tissue) :

Glandular breast : (Dense breast) In young woman, where breast contain large amount of glandular tissue.
Adipose breast : (Fatty breast) In older women, where most of the glandular tissue has involuted.
Involuting breast : When mixture of soft tissue and fat density present.
Less glandular tissue and more fat
Young females are less cooperative
Young breast have dense tissue
Because of less fat content