There are several possible reasons :
There are cluster members with a lower software version on this subnet / VLAN
[member with higher software version will go into state 'Ready'].
The number of CoreXL FireWall instances on cluster members is different
[member with greater number of CoreXL FW instances will go into state 'Ready'].
Note: This applies only to R80.10 and lower versions.
The ID numbers of CoreXL FireWall instances and handling CPU core numbers on cluster members are different.
On Gaia OS - Linux kernels on cluster members are different (32-bit vs 64-bit)
[member with higher kernel edition will go into state 'Ready'].
On Gaia OS - Cluster member runs in VSX mode, while other members run in Gateway mode
[member in VSX mode will go into state 'Ready'].