Twitter follow button not appearing correctly on some pages

This is a strange problem.
I use the code from Twitter to put a Follow button in my posts and sidebar.
On the front page, everything works fine, but when I click on a particular post, it no longer renders properly (text only, although the link still works).

eg. Compare:
http://subcontinental.net/
and
http://subcontinental.net/2015/01/04/the-year-vladimir-putin-tore-up-the-rule-book/

Can anyone help? The same thing happened with the Eighties theme and the Harmonic theme I’m currently using.

Many thanks
Eric