In addition of WoolWell’s answers, you should double check the send/receive settings. Might be manual send/receive option is already turn off.
If the above given solution does not help, then go and repair the PST File by using scanpst.exe. Corruption in Outlook PST file or in MS-Outlook can create such problems.
Hopefully it will resolve your issue.
If scanpst.exe does not helps in resolving send/receive issue, then go for a SysTools Recovery Software.
Thanks Clark Kent