Lowrance Hook2 7x TS Not finding bottom

the Hook2 5 TS on my Outback occasionally loses the bottom but my 'workaround' is to just manually set the range for sonar (which is super easy once you've done it a few times) to the appropriate depth and within a few minutes it locks on the bottom again.  On waters with fairly consistent depth I often don't even bother to switch back to auto range, just no need. 

no other issues with my Hook2 5 TS in 5 years of fairly heavy use.