Tente usar o endereço IP em vez do ServerA / ServerB. Dos documentos do RHEL :
trecho
14.4.6. mount command fails with NFS server failed error.
mount command fails with following error:
mount: mount to NFS server '10.1.10.11' failed: timed out (retrying).
Perform one of the following to resolve this issue:
Disable name lookup requests from NFS server to a DNS server.
The NFS server attempts to authenticate NFS clients by performing a reverse DNS
lookup to match hostnames in the volume file with the client IP addresses. There
can be a situation where the NFS server either is not able to connect to the DNS
server or the DNS server is taking too long to responsd to DNS request. These
delays can result in delayed replies from the NFS server to the NFS client
resulting in the timeout error seen above.
NFS server provides a work-around that disables DNS requests, instead relying
only on the client IP addresses for authentication. The following option can be
added for successful mounting in such situations:
option rpc-auth.addr.namelookup off
Portanto, com base na sua mensagem de erro, acho que você está tendo problemas com o fato de os servidores não conseguirem resolver um ao outro por meio de seus nomes de host.