I get the option to reply to a post when logged in as administrator, but it's missing when I am logged in as a member. I notice other members have replied, so I know it's possible (I am coming in after the forum had been set up). I've checked and rechecked all of the permissions for the two forums in question, and they are correct - they are set for "full."

However ... when I go to my forum profile as a member (of either of 2 forums in one instance of SimplePress) and look at my personal permissions, I do NOT have full access. I don't have permission to reply or many of the other options. 

Is there a setting I'm missing? Am I doing something wrong?

So are you meaning with two different user accounts? One for you role as the forum admin and one as an ordinary member?

First thing to do would be to go to the forum admin > Toolbox > Housekeeping and run the task to Reset the Auths Cache. See of the rebuild sorts it out...

I ran the housekeeping tool and it seemed to work for one of the forums, but not the other one. I dug in a little more and finally ... finally found one of the obscure places permissions were set and someone had set the permissions for that specific user group to limited instead of full. Everywhere else it was set to full.

Thank you so much for at least getting me to dig more!!

That's good news. Thanks for updating the topic and glad that you tracked it down successfully.