What deployment method are you using? If it is WCCP, you will most likely need to change your hashing input to use like destination IP for load balancing to ensure a more even distribution.
Can you post screenshots of your proxy HA settings? (configuration > Proxies)
It could be that things are misconfigured and load sharing is not working.
Also from the CLI, could you post output of "mfend-lb -s" this will tell us if things are working as well.