I was facing the same problem during testing on mobile devices and this trick worked for me
<a type="submit" class="btn btn-primary" data-toggle="modal" href="#myModal">Submit</a>
Change the button to anchor tag it should work, the problem occurs due to its type button as it is trying to submit so the modal disappears immediately.and also remove hide from modal hide fade give
<div class="modal fade" id="myModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel" aria-hidden="true">
Hope this would work for you .