Try running a memory diagnostic tool with both the memory modules installed - click here to get the tool. It can take a LONG time to complete.
Make sure all your cables are secure inside the case & seat the main power cable firmly & fully into it's slot.
The second memory slot may be damaged & so making an insufficient/intermittent connection.
When you post again please provide your OS, type of memory, PSU power output, & mobo make & model.