Lost conversation group when shift from outlook 2011 to outlook 365

Hi,

I have been using outlook 2011 for mac. Days ago, I moved to office 365. When I installed new version of outlook in office 365, it started to automatically import my old outlook 2011 emails. Everything done pretty good. However, I found a big problem. All conversations group of my emails are gone. All emails became individual email and can not be grouped by conversations. When I started to receive new email in new outlook, only these new coming emails got grouped again. All old emails just lost the conversation group.

Is there any way to repair it and get all past emails grouped again?

Thanks!