With Google Chrome browser, the zoom must be at 100% for drag and drop to work. Check the zoom level under the Chrome logo dropdown in the upper right.
Possible workarounds:
1. Set the zoom to 100% in Chrome.
2. With Chrome, use cut and paste, instead of drag and drop.
3. Use a different browser, such as Internet Explorer or Firefox.
Please note: only certain lists can be rearranged in this way. Example: requirements, test cases, and test steps. The other lists, such as incidents and tasks, can be sorted by clicking the arrows on the column headings.