You may have to tinker abit, the ACOM on-line tree program has its quirks with marriage events.
I would suggest you turn off auto-sync in FTM, choose manually sync. This prevents constant syncing which is not needed and can cause all sorts of problems.
In your tree in the ACOM on-line tree program.
-Check both profile pages of the couple, if there are any "marriage" events in either, remove them. You want to start with a clean slate. Removing the "marriage event" will not remove the citation/document from the profile, you'll re-link/attach this later
-Now choose *add a fact* on either profile page, choose marriage, and follow the prompts to link to the other party. You can also enter the date and place at this time.
-When you return to the overview of the profile page, you should see in the time-line "Marriage to John Smith", and on the other parties page, "Marriage to Jane Doe".
-If you have a citation for the marriage already linked/attached to the profile(s), you can now relink it to the event. There are some threads on this board about how to move and link citations in the member tree program. This is something you should learn, it comes in very handy, particularly when dealing with profiles that have multiple marriages.
-Also note that no two (ACOM) databases link/attach exactly the same, the more you work with specific databases, the more you learn about how they react when linking them to profiles in your tree.
The sequence to follow is to always create a "Marriage" fact before linking/attaching a citation.