two things, one is the lake keeping temps down, and the other is that
clay warms more slowly than other types of soils. Then, of course,
there is the amount of sunlight they get (full sun is good), whether or
not they are near a wall (wall is good) or at the bottom of a slope
(bottom is bad). if you mulch before the soil has warmed up, that is a
major difference. On the other hand, your plants probably last longer
in the fall.
The only thing you can really do to help them is make sure you mulch
after june 15. you can of course dig them all up in october and replant
them in a raised bed. Most of them will not mind it. That will speed
their emergence by a lot because a raised bed warms much faster.