Except for loufly, it seems most of the responses are about getting down in a deep lake. I am not disregarding that, but would like to point out a couple of additional thoughts, based on the idea of looking at the contours of the lake and water movement. My general thought with your recent lack of success is that you are fishing in the same place as before, but the majority of the food or the best water (cover, temp, oxygen) is now somewhere else on the lake. Was your success right after ice-out when the water near shore was cold? If you fish from shore (as it seems from your description) then you will have challenges, maybe it is time for a watercraft.
Anyway - lakes do have some characteristics in common with rivers that people often don't think about; namely current and the 'backside'. They also have structure in the form of rocks, underwater shelves and temperature stratification. Fish seek structure for security in both rivers and lakes.
Current is caused by the wind, and the fish will be on the backside of points where the food is drifting in the foamline. The wind also blows food toward the leeward shore and stirs up sediment there, exposing more food (or at least minnow food: minnows = bait for big ones). They will also be on the backside of anything that produces shade (especially when they are resting) or the backside of weedbeds where food will become exposed. They will be near rocks, weeds, shelves or logs that provide cover. They will also be in a place with the preferred temperature. The wind can move warmer water to the leeward side of the lake, but also the north side of most lakes warms before the south side (in the Spring). So maybe they have moved to warmer water, or maybe they have moved from where you had success because the water is now too warm (for either them or their prey).
If you were a nomadic fish in that lake, where would you set up the most comfortable and convenient camp for the next week (until conditions change again)? Maybe they’re deep but maybe they have just moved camp.