There are many kinds of besan available in the market. Depending on the quality of the besan and how old the besan is, it is possible that the besan that you are using never reaches that paste-like consistency even after roasting for 25 minutes. Do not worry, keep roasting and do the taste test. If the besan has nutty aroma, doesn't taste raw and has turned golden-brown in color it is done. Yours ladoos will be just fine even if the besan never turned into that paste like melty consistency.