Checkbox value not submitting - php

Question

EDIT
The form, and checkbox submits completely fine in Internet Explorer 8. In IE 10, and Chrome 34.0 it doesn't get submitted
Ok, this one has me stumped. I have a form, each form contains one question for a quiz, its possible answers, and a checkbox to select which is the correct answer.
<form action="/department/update_quiz_question/1/1/5" method="post" class="form-horizontal form-bordered form-row-stripped" id="question_form_5">
<tr id="question_5">
<td class="col-md-4">
<textarea name="quiz_question" class="form-control" rows="4">What letter precedes B?</textarea>
</td>
<td class="col-md-5">
<div class="input-group ">
<input type="text" class="col-md-5 form-control" name="answer[]" value="A">
<span class="input-group-addon">
<input type="checkbox" checked="checked" name="correct_answer[]" value="A">
</span>
<span class="input-group-btn">
<button class="btn red btn-delete-answer" type="button" data-question-id="5" data-answer="A"><i class="fa fa-trash-o"></i></button>
</span>
</div>
</td>
<td class="col-md-3">
Add Answer
Update
Delete
</td>
</tr>
</form>
When I submit the form, it goes to a Codeigniter method which only contains:
print_r($_POST);
var_dump($this->input->post('correct_answer'));
As you can see, the checkbox is most certainly checked. It is also checked visually (IE there is a tick in the box).
When I submit the form, it goes through a jquery checking script. If all passes, jquery submits the form.
Just before I submit the form, I log the value of the checked checkbox, using the following:
console.log($(".input-group input[name^='correct_answer']:checked").val());
Which, as you would expect, outputs A, or whichever answer is ticked. So I know for certain the checkbox is ticked.
However, Codeigniter outputs the following:
Array ( [quiz_question] => What letter precedes A? [answer] => Array ( [0] => A ) )
bool(false)
The array is from the raw $_POST data. bool(false) is outputted from $this->input->post('correct_answer');.
As you can see, the checkbox value for correct_answer is not even displaying in raw $_POST data.
I can't understand what I've done wrong. I have an identical form when I create a new question, and that works perfectly fine.
Can anyone see any obvious mistakes? Or am I doing something wrong?
Many thanks
EDIT
This is what I've tried so far:
Removing all the span elements around the checkbox, leaving just the text input and the checkbox input - Still not checkbox value in the post data
removed the square brackets around the checkbox name - no post data
renamed the checkbox name, no post data
Submitted the form straight away instead of passing through a jquery check first, no post data
Changed it from a checkbox to a radio button - no post data
changed it from checkbox to text, - works, doesnt work when its a radio or checkbox

Answer 1

Your HTML code is invalid – you can not have a form element as child of table.
Either put the whole table into the form, or the whole form inside a td element – these are your only two options.¹
Different error correction algorithms for faulty HTML most likely explain why you see your current code working in a certain browser, but not in others.
¹ Well, if you want to stick to a table anyway – as you said in comments, moving away from using a table resolved that structural issue as well.