dns query gives inconsistent results

when I run .server_config PTR query  it can succeed or fail for same IP address.

We have 3 DNS servers defined and I wondered how the celerra uses them, does it go to the first one every time or is it a random selection.

It makes no sense that a query can succeed , then fail     see below eg

$ .server_config server_2  -v "dns query PTR=10.235.232.16"

server_2 : commands processed: 1

command(s) succeeded

output is complete

1436147816: LIB: 6: 10.235.232.16

1436147816: LIB: 6:  Type:PTR       TTL=86400 s  dataCount:1

1436147816: LIB: 6:   ora0005-nas.telecom.co.nz

1436147816: LIB: 6: ---

1436147816: ADMIN: 6: Command succeeded:  dns query PTR=10.235.232.16

$ .server_config server_2  -v "dns query PTR=10.235.232.16"

server_2 : commands processed: 1

output is complete

1436149924: ADMIN: 3: Command failed:  dns query PTR=10.235.232.16

Error 4020: server_2 : failed to complete command

Can anyone explain this behaviour please?