If you seem to be missing emails in an email thread in Office365 web mail @ outlook.office365.com, this is Outlook's default "conversation view", which is similar to Gmail in that multiple emails on a single email thread are collapsed into a single line item entry in your Inbox view. You can expand the conversation view by clicking the blue arrow to the left of the message, or you can disable Conversation View in Outlook Office365 web mail entirely by performing the following while viewing your Inbox:
Office365 Outlook web mail "missing" emails in an existing email thread
1. Click "Filter"
2. Click "Show As"
3. Click "Messages" (instead of the default "Conversations")