LTM/GTM HA Configuration

Hi, I would like to seek your help regarding my implementation for F5 Redundant GTM/LTM set up.

I have 1 pair of LTM/GTM appliance running as Active/Standby HA Pair with GTM enabled on both. When I create a Server under Data Center A and configure the active LTM as a device, all goes well. However, when I try to add the secondary device (which is the standby F5) to proviude redundancy, some of the virtual servers goes down.

Any advice why this scenario is happening plsss?

Thanks and kind regards,

Jeff