DNSSEC validation option in BIND 9.10

[Tom Yard]

Hi people, I have two BIND 9.10.3 servers with DNSSEC validation enabled,
one in one client and the other in another client.

Both BIND have the same configuration lines relative to DNSSEC validation:

dnssec-validation auto;
dnssec-enable yes;

and both has the current and future key in bind.keys.

But I have a problem: in one of these BIND servers, when I execute certains
manual query, for example:

dig www.pami.org.ar
dig www.anses.gob.ar

I get:

Host www.pami.org.ar not found: 2(SERVFAIL)
Host www.anses.gob.ar not found: 2(SERVFAIL)

But if I change the line:

dnssec-validation auto;

for this:

dnssec-validation yes;

the DNS queries are succesful. But if I maintain the line
"dnssec-validation yes;" then the DNSSEC validation desn't work anymore.

Can you explain me reason for this behaviour? I want to have DNSEC
validation, but if I put "dnssec-validation auto;" the DNSSEV validation
works OK but I have resolution problems with certain domains.

Thanks a lot, bye.

Tomy