however when i do any changes, i check the status for sync-only group it says 2 device are consistent but yet the changes i made hasn't been sync over and the top left status is still pending changes.what object did you change? it was *not* failover object (object which floats between unit when failover), is it?
Sync-Only
A Sync-Only device group contains devices that synchronize configuration data, such as policy data, but do not synchronize failover objects. A maximum of 32 devices is supported in a Sync-Only device group.Manual Chapter: Understanding Device Groups
http://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/bigip_redundant_systems_config_11_0_0/5.html
Nimbostratus
