So you said new transducers in OP. What models?
PDRT-WBL has external temperature suitable for thru-hull.
PDT-WBL has temperature suitable for outside hull(TM or transom).
PD-WBL does not.http://www.lowrance.com/en-US/Products/Transducers/PDRT-WBL-DepthTemperature-Transducer-en-us.aspx
If correct model of transducer, then check Settings or attempt to swap HDS unit to determine in HDS or transducer issue.
Do you see good sonar reading which would tell that HDS does see the transducer?
Check following settings: