Could be several reasons. Can you please describe the switch configuration?
If on a Cisco switch can you do for example:
# show stack
# show interfaces port-channel 1
Check you have VPc (Cisco) in place if you are using multiple switches.
Vlag for Brocade I think.
Do you have more than two Ethernet switches here (on this Veth port)?
If not then perhaps you have the active interface on one switch and the other 3 on another switch.
Without VPc it will just persistently use only paths on one switch and pretty much ignore the other switch/path.
My question about number of switches being used is because it would normally be that you have only TWO interfaces being used...on one switch - the other 2 being idle on the other switch.
and by the way, it is not DDBOOST backups, it is CIFS backups. the LACP is accross 1GB ports.
I assumed it was probably not DDBoost or you would probably be better advised to use ifgroups and not LACP.
My response was based on the likelihood you did indeed need LACP for non-DDboost backups.