First of all, make sure you exported the loop with the ending natural sound decay (or reverb). Once in Wwise, make sure you have correctly set the time signature and tempo from the Property Editor (General Settings).
In the Interactive Music layout, the segment should now be correctly adjusted to the grid. Make sure you place the entry and exit cues correctly (green and red flag, should encompass what should be repeated, not what's before or after, such as the final reverb).
Should work now. If not, let me know.