In case the documentation or steps shared above are not working, I strongly suggest you creating your own question in our communities channel for further assistance. At this point, I will proceed archiving this support ticket."
I experienced this problem too but there is no solution because the problem is not on Splunk side. FMC stops sending data or sends extremely slow to the socket at that times. If restart on Splunk does not help, restarting e-streamer service on FMC should work.
You can create a bash script to check data delay and send restart to estreamer app on Splunk.
Another better workaround is if you are using V6.4 on Firepower, using syslog.