I looked at some of the topics that have been migrated over and there is no reply button. On post that were created here there is a big blue Reply button.
I see that too. Also, a reply must be longer than 20 characters. So, I added this, to comply, lol.
I think you can reply to new messages but not the migrated ones from the old site.
Good catch. I will take a look at this issue and reply back. Thanks for reporting it.
Yep. I have also seen this. Working on it. As I think it was @Gramps said, it is like moving into a new house. Need to try and find everything.
I checked through several of the old threads and the Reply button is now showing.
Nice job Irv tracing that down in the new house.
@Gramps I should have seen this earlier. But thank you for the thank you. 
I was in IT for years, sometimes it takes the second set of eye to see a problem. Been down that same road many times. Happy to help!