Cellular companies in India often don't have all towers 4G enabled. If tower near you is not 4G enabled then you will not connect to 4G network. In other case limited number of users can connect to one tower even if companies have multiple towers near your location lets say one tower is 4G enabled and other only 3G, once all slots of 4G are connected next subscriber, trying to connect, will be redirected to 3G enabled tower.
