Tasks can sometimes "get stuck". It may not happen to everybody, but it happens to quite a few of us.
Look under "Awaiting tasks", and if its not there, check "Completed tasks" - it will say "Now you can re-apply" on the right hand side of the screen on your current level, and those words are clickable, and will give you the task coins.
If none of those work, deleting cookies and Internet history should do the trick.
It was actually in awaiting tasks, so thank you for the advice. I clicked on the name of it, and it took me to the task, and then I clicked on "Reward". It is now in completed tasks, so hopefully will show up again.