I had this problem when trying to reply to the TurboCad printing problem thread.
I’d viewed the thread whilst logged out, then logged in and re-navigated to the thread only to find no reply box. I then came to this thread and found that the reply box was visible. So, I returned to the TurboCad thread, where the box remained absent. However, once I clicked my browser refresh icon, the reply box appeared.
Presumably this must be a caching issue, perhaps where previously loaded pages are not completely reloaded when returned to?
Hope this helps someone else.
Regards Tom (using Firefox with Ubuntu 22)