Backup Job throughput not changing dynamically

I have created client backup to run with 4 streams, Initially 2 streams started with 18MB and 20MB throughput and other 2 streams started little late with throughput 4MB and 7MB; that seems to be good throughput as the combination of those jobs come to a good amount. But after completion of those first 2 streams(18MB,20MB) other 2 streams were running with same old speed. I see this is the case for any other client backups too. Basically job throughput is not getting dynamically increased based on the server/network load. how do we fix this issue?