Hello there ,
It seems as though your lab setup could have a flaw , would you be able to share your physical cross connects lab diagram as well as your logical diagram vlans etc.. ? there could be many reasons your servers did not communicate correctly with the firewall when firewalls switched to the standby.
Below is a URL with a straight forward A/S deployment , go over this link, if you are introducing two switches in your design you need to keep in mind that if Active FW connects to one switch and the Standby to another switch both switches must be connected and they have to be able to be aware of VLANs you have allocated for each of the firewall interfaces you have configured them with - proper cross connects and proper VLAN assignments in the switches is key to for proper fail-over events and systems to continue their communications
https://www.cisco.com/c/en/us/support/docs/security/asa-5500-x-series-next-generation-firewalls/91336-pix-activeactive-config.html
Additionally I have attached an basic site cross-connect diagram to help you with the physical aspect. Again, if you do have configurations of your switches and firewall we could sanity check to see where is the flaw.
Jorge Rodriguez