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Even if conditions were

Even if conditions were exactly the same for both squashes, the results may not be the same.

Zucchini seed germination requires a minimum soil temperature of 60°F, according to several sources, with a pH of 6.0 to 7.5. BTW, one even advices starting both squashes in the ground, to avoid transplanting failure.

It's difficult to know eactly what went wrong. See what happens with the existing seedlings, and try again and, if you do, save a couple of seeds to set directly into warm soil in the garden.

Good luck!