Either the secondary DNS server's zone file/database doesn't have the required records or the client lacks the secondary DNS server in it's network configuration. When validating the DNS records, ensure both the SRV and A records they resolve to are working.
No. The user's PC/laptop/mobile device needs to know about the secondary DNS server. Try doing a forward and reverse lookup for all of the records on the secondary server. Either the client doesn't know about the second DNS server or a record is missing.