VS 2013 Update 3 (not sure if others are affected)
If you type the following slowly in a django template, one character at a time (on the same line):
You should see completion list.
But if you type at regular speed, especially if you type the space quickly after the %, then the completions won't come up. You will have to CTRL-SPACE to force it to come up.
I'm using details.html from the Polls Django Web Project template for my repro.