How to get back 3G speed in BSNL(3G recharge done)?

I have BSNL 3G sim which I was using 3G plans. But last month I recharged with 2G plan(that time I got 2G speed). Yesterday I recharged with 3G plan again but I am getting 2G speed only. 2G leftover data has been added with 3G data. I have selected UMTS network only. Please help. Thanks in advance.