Hi,
If NAT-ing did not resolve the problem, more likely it is not a routing issue on the server LAN.
Does Site-A have the necessary policies to allow this traffic?
If the config looks good, you can collect a simple debug on Site-A FW:
undebug all
clear db (will clear the exisiting debug data)
set ff src-ip <bgroup ip of the Site-B firewall> dst-ip <ip of the server>
set ff src-ip <ip of the server> dst-ip <bgroup ip of the Site-B firewall>
debug flow basic
<<ping server from siteB FW bgroup>>
<<Press Esc once it fails>>
get db st
The log printed by the last command will give an idea about traffic processing