LAGs hash traffic across ports with an algorithm based on the source and destination addresses. This can be based on either the L2 addresses, L3 addresses, or a combination of the L3 addresses and L4 port numbers.
If the algorithm is not specified when enabling the sharing, default L2 load sharing is used which only distributes traffic based on Mac addresses and ports.
Any traffic between the same source and destination will be sent on the same port. If this is the only traffic on the LAG, it will all be sent on one link.
Changing the hashing algorithm to use IP information or IP+TCP/UDP information may improve traffic distribution.
Adding more ports to the LAG may also improve behavior, but this is very traffic dependent.
The command to enable sharing with specifying the algorithm is done using this command:
enable sharing <port> grouping <port_list> algorithm address-based L3_L4 [lacp]