Never fished the lake in question - however...

when I'm searching for big 'uns, I just try to differ presentations and depths to see where they are & what the're feeding on. Sometimes they are cruising the really shallow water (4-8ft) looking for meals, other times they are really slow and sluggish in 30+ ft of water. Unfortunately, no hard and fast rules.

However, as a general rule, if you make the free meal larger, the larger guys are more willing to burn energy in attempt to catch and consume. Try moving up to the next size hook (not sure if you're using jigz ?!?) and up the size of the bait. See what happens. Keep track of your catches. Move up again and see what happens from there. Basically just T/E but track your results and you'll learn the lake.