The layer that matters
By midsummer a deep lake stratifies into three layers. The warm upper layer holds oxygen. Below it sits a narrow band where temperature drops fast, and beneath that, cold water with almost no dissolved oxygen at all.
Fish cannot live in the bottom layer. If your electronics show fish at 40 feet and bait at 18, the fish are not at 40 feet.
Finding it
Watch for a hard clutter line on sonar. That is suspended plankton stacked on the density change. Fish the few feet above it, not below.
When it breaks
Fall turnover mixes the layers again and oxygen returns to depth. For a week or two the lake fishes badly, then it fishes better than it has since spring.