In a single-tier vCenter Configuration Manager setup it is often forgotten to restrict the Microsoft SQL Server memory usage, this is essential as some of the VCM and MS SQL components might run out of memory, as per default MS SQL Server might take all memory.
To change the memory settings:
Connect to the VCM collector (your VCM server)
Open Microsoft SQL Server Management Studio
Login to the collector server MS SQL database
Right click on the collector server and select properties
Select the memory from the left hand page selector