Form in wordpress isn't reading all checkboxes on first submit

Question

When submitting forms on a wordpress page with multiple checkboxes, on the first submit, when I print_r($_POST), the checkbox array shows two items without their values. Occasionally it works.

Here's the code:

<form action="<?php echo htmlentities($_SERVER['REQUEST_URI']); ?>" id="assessment-form" method="post">
<ul>
        <li>
          <input type="checkbox" name="Answer[]" value="Checkbox value" />
          </p>

          <p class="question"><label>Checkbox value</label>
          <input type="checkbox" name="Answer[]" value="Checkbox value" />

          </p>

          <p class="question"><label>Checkbox value</label>
          <input type="checkbox" name="Answer[]" value="Checkbox value" />

          </p>

    <input type="submit" name="submit" value="submit" />

</li>
</form>

Any help would be appreciated!

Edit: I'm referring to just the checked checkboxes