have you checked your bbpress forum settings page, or looked at the bbpress instructions ?
https://codex.bbpress.org/forum-settings/
Lock post editing after
Set this to the time (in number of minutes) to limit post editing. Once the time limit has been reached the forum post no longer be available for editing to forum participants. Administrators and bbPress Moderators can override this setting at any time.
This may not be the same problem, but the submit button (after editing) disappeared after a while, when our forum topic titles got too long. If the list of destination topic titles in the drop down menu after selecting “move” or “merge” were especially long, the submit button would be pushed off screen. To remedy this, we had to go in and rename the long titles shorter.
