There is no way to specify the flow of traffic to multiple anchor wlc's. The foreign wlc will determine which guest anchor it will use if multiple have been configured in the mobility group and in the ssid.
Is this happening to users on both guest anchors or not. Is the users roaming between ap's on different foreign wlc's? If so, then you must make sure the data and control path between the two guest anchors are up and also the foreign wlc's. What you can do is take down one of the guest andchors and see if there is an issues with one of the guest anchors.
sorry for the late reply but i was busy with other things.
I disabled one Anchor as you suggested but the problem didn't disappear.
The problem only happens to wired Guest access users not to the wireless ones. When i check the local controllers i can see the mac-address of the wired client on both Controllers. The problem seems to be there.
I have disabled now the wired guest access on one local controller to see if the problem is gone. will keep yo updated.