Answer: It could be the cool temperatures, but raised beds usually warm up more quickly than those that are at ground level. When you filled the bed, did you tamp it gently or allow it to settle before planting? Fluffy soil is great, but if there's too much air space in the soil, seed and soil don't have enough contact, and seeds can't soak up enough moisture for germination. Water will run right through it. Try digging up a few of the corn seeds to see if they've started to sprout or not. You may need to replant. When you do, make sure the soil is moist enough - will hold together when squeezed in a ball, but not so wet that water drips from it. You can also check with the source of the garden mix to see what they can tell you about the soil contents.
Just an FYI - though I doubt this would discourage seed sprouting - if the old deck wood is pressure-treated, it contains poisons (usually copper aresenates) that are toxic to all living things, even plants. Some people say they shouldn't be used in gardens at all because the toxins may leach out and kill plants. Others believe the copper arsenate is well enough fixed in the wood that it won't cause problems. There's scientific evidence that supports both views.
Best of luck!
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