Lowrance LCX18 combo. Poor transducer signal and fish sign at 150-200 m and over

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I am not happy with my Lowrance transom mounted transducer (LCX18 combo) as i can not see fish sign at depths over 150m. I consistantly loose signal when i back into the current. Is the transducer the problem or is it the set up. I have a 6.2 m aluminium pontoon boat with a 18 degree transom. Please offer your good advice!