Citrix ADC / NetScaler Gateway STA server shows DOWN for one Delivery Controller only

Hi everyone

I would like to know the best troubleshooting steps for this situation.Hi everyone,
We have a Citrix Virtual Apps and Desktops environment with Citrix ADC / NetScaler Gateway.
On the ADC Gateway vServer, we configured two STA servers, which are our Delivery Controllers:

DC1 - Delivery Controller / STA - UP
DC2 - Delivery Controller / STA - DOWN
The issue is only with DC2. DC1 works normally.

What I already checked:

- Citrix services on DC2 are running
- Delivery Controller DC2 is active in Citrix Studio
- Firewall rules look OK
- No obvious network block between ADC and DC2
- DC2 is reachable from the network

In ADC / NetScaler, the STA status for DC2 is still showing DOWN.

I would like to know the best troubleshooting steps for this situation.