This may related to the defined outbound policy (Loadbalancing method) or the WAN connection issue.
Please refer to the URL below to isolate the issue:
http://www.peplink.com/knowledgebase/speed-test-tool-for-combined-download-speed-in-multi-wan-environment/
http://www.peplink.com/knowledgebase/combining-the-bandwidth-of-all-wan-links-3m-3m-6m/#LB