Changes in current and temperature. These go along with depth. Water moves around, no matter the container. Are there inlets and outlets to the lake? Does the wind move it in a particular way?
Take a few minutes to study the lake. Watch it across various times of day. And, for crying out loud, vary the depth of your flies, if you're fishing subsurface. Once you find where fish are hanging out, you'll have a lot more success.
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