If you are not able to get tickets from Coimbatore Jn for this train, try from Palakkad junction which is the previous stop. This falls under GNWL (only if u book till Guntur Jn). Same tip can be followed during tatkal booking also. Tatkal from Coimbatore will only be 43 in sleeper class where as from palakkad it will be around 238.
Note: If u dont book tickets till Guntur Junction from Palakkad, it will come under PQWL which is difficult to get confirm. In that case u can book from Coimbatore otself which falls under RLWL.