>> Sometimes it completely fails to connect or allows connection and then later on disconnects the user
It's more likely a poor network conditions cause user disconnected, you can check a few things as below:
1) Compare the network settings between "completely fails" and "allow connections and then later disconnect...": network cable connect or wireless connect, proxy enabled or disabled, etc.
2) Check if the same things happens to different clients, like Horizon Windows client and HTML Access client.
3) Check with your IT department to see if the connection is blocked by your company security settings.
