You for sure have it going into the main placeholder? Do you have any renderings that do not have a placeholder specified?
What version of Sitecore are you running? Per the link @CoeuerDusite provided, it is a known issue:
To address the issue, apply one of the solutions below:
Always specify placeholder name for all presentation elements.
Upgrade website to Sitecore 8.0 rev. 150427 (Update-3) or newer, where this issue has been fixed.
Apply the patch as described below:
Put an Sitecore.Support.380995.v*.dll assembly into the \bin folder;
Put Siteocre.Support.380995.v*.config file into the \App_Config\Include folder.
Appropriate files can be retrieved from an archive, depending on the Sitecore version:
Sitecore 6.5 - 6.6: Sitecore.Support.380995.v1.zip - https://kb.sitecore.net/~/media/2C9A39977D8349EEB378F89245DC6D61.ashx?la=en
Sitecore 7.0 - 7.5: Sitecore.Support.380995.v2.zip - https://kb.sitecore.net/~/media/AE61B2778ACC4C108872F5F63519A67E.ashx?la=en
Sitecore 8.0: Sitecore.Support.380995.v3.zip - https://kb.sitecore.net/~/media/92301E04A678428CA7D13354B5F2E8FA.ashx?la=en