When Splunk starts up it assesses the host for the available memory and CPU count.
The number of CPUs present at startup dictates how many searches can be run in parallel by Splunk.
If you add more cores, Splunk will not know this, and will limit itself to the number of cores it had when it was originally started.
Memory is less likely to be limiting, as Splunk will just let the OS handle memory allocation.
The correct (actual) CPU count and installed memory figures will also be incorrectly reported to the monitoring console - whilst this is probably not going to cause any "issues" it could be confusing.
I would suggest that you should restart Splunk if you change the allocated resources.