I am currently applying to graduate school and trying to gather some solid letters of recommendation. I already contacted three professors (almost 1 month before application deadline) and 2/3 gladly accepted. The third professor that didn't reply at all already wrote me LoR one year ago so I don't see why he wouldn't like to do it again. How do I ask again without being pushy? Can someone help me what should I write in follow up mail?
There is probably no template, and it would probably sound insincere anyway. Apologize for asking again so soon, but remind them that there is a deadline. If they can't write for you or don't reply, move on.
But, you might also ask their department whether there is some reason they can't reply.
And look for an alternative.