India and the United States, both have volunteer armed forces of about the same size. But India, with more than three times the population, has a hard time attracting sufficient new recruits. The U.S. has more recruits than it needs, while India has shortages. The reasons are partly economic, and partly due to a high number of illiterates (about 39 percent of the population) in India. In the United States, the military pay is competitive with the civilian economy. In India it is not, at least for the kind of people (literate and healthy) the military is looking for. In the last two decades, the Indian economy has been booming, and the kind of people the army used to go after, are taking better paying, and less arduous, jobs in the civilian sector.
While the army has a hard time, for the more technical air force and navy, the situation is much worse. These services insist on recruits having the equivalent of the American high school education (much less common in India), with heavy emphasis on science, including familiarity with computers. The screening process takes two or three days, which consists of written and physical exams. If you make it, the initial enlistment is for ten years. The basic training is rough, and not everyone makes it.
The shortages are worse for officers, with India only able to obtain about 80 percent of needs. The reason is the same as for the enlisted troops; better civilian career opportunities. But the Indians have wisely maintained their high entrance standards, and have been increasing the defense budget enough to make officer pay more competitive. This problem will not go away, and as the civilian economy grows, the number of people willing to volunteer for military service will continue to decline unless the pay and benefits get competitive.