OK, so let's go back to basics here and confirm the configuration;
1) You have a single VLAN configured on both devices using the same tag number, Source Check and Failsafe are both disabled, assigned (untagged) to a single interface on both devices, the 'physical' interface used is the same on both devices and configured for the same speed and duplex settings
2) You've configured both devices to trust each other and each appears in the other's Peer List
3) You've created a Device Group and assigned both devices to it, on both devices and enabled Network Failover
4) You have a non-floating Self IP configured on both devices and assigned to the single VLAN, with Port Lockdown configured as Allow Default and assigned to the traffic-group-local-only traffic group
5) You have a floating Self IP configured on one device, assigned to the VLAN, with Port Lockdown configured as Allow Default and assigned to traffic-group-1
6) In System > Platform the root folder Device Group is your device group and the root folder Traffic Group is the one used in 4)
7) In Device Management > Devices > 'name' > Device Connectivity > ConfigSync you have the local non-floating IP address specified, on both devices
8) In Device Management > Devices > 'name' > Device Connectivity > Failover you have the same local non-floating IP address and port specified, on both devices
9) In Device Management > Devices > 'name' > Device Connectivity > Mirroring you have the same local non-floating IP address specified, on both devices