Imported my party from Curse, and my human 9th level Fighter/13 level Thief has been unable to land a backstab at all in Secrets.

He is wearing +1 leather armor and using a broadsword.

In Curse, I was able to backstab successfully with a bit of preplanning, not so in Secrets.

Any tips, is the broadsword the problem, and as soon as I find a longsword all will be well?

You should be be able to backstab with Broadsword and Leather +1.

Does the target turn around when you try to backstab?
If so you are doing things wrong.

Are you trying to backstab targets larger than one square?

DOH! Minotaurs and Hill Giants, of course... Thank you, at least now I understand what the problem is :p