Drupal drag-able blocks issue

Question

Druapl offers great functionality whereby blocks can be dragged up and down to reorder as shown below.

However, on a couple of projects I've been working on they have disappeared during development )as shown below) but I have no idea why! It must be something that I am doing wrong but for the life of me I can't work out why.

Answer 1

I removed the Google CDN jQuery link and normality is ensued.

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.4/jquery.min.js"&gt;&lt;/script&gt;