Bind9 won't resolve some domains. Why?

Question

linux

Debian GNU/Linux 9 (stretch), Linux 4.9.0-7-amd64, x86-64
bind9 Version: 1:9.10.3.dfsg.P4-12.3+deb9u4
PRO zone root servers do not resolve.
[email protected]:/etc/bind# dig 127 .0.0.1 nic.pro +trace
; <<>> DiG 9.10.3-P4-Debian <<>> 127 .0.0.1 nic.pro +trace
; (1 server found)
;; global options: +cmd
. 35578 IN NS b.root-servers.net.
. 35578 IN NS e.root-servers.net.
. 35578 IN NS m.root-servers.net.
. 35578 IN NS h.root-servers.net.
. 35578 IN NS l.root-servers.net.
. 35578 IN NS k.root-servers.net.
. 35578 IN NS i.root-servers.net.
. 35578 IN NS j.root-servers.net.
. 35578 IN NS g.root-servers.net.
. 35578 IN NS f.root-servers.net.
. 35578 IN NS c.root-servers.net.
. 35578 IN NS d.root-servers.net.
. 35578 IN NS a.root-servers.net.
;; Received 1136 bytes from 127.0.0.1#53(127.0.0.1) in 0 ms
pro. 172800 IN NS a0.pro.afilias-nst.info.
pro. 172800 IN NS a2.pro.afilias-nst.info.
pro. 172800 IN NS b0.pro.afilias-nst.org.
pro. 172800 IN NS b2.pro.afilias-nst.org.
pro. 172800 IN NS c0.pro.afilias-nst.info.
pro. 172800 IN NS d0.pro.afilias-nst.org.
pro. 86400 IN DS 47221 7 1 00CBFCBDC9C6E659FB2385B3982E93E98D71052D
pro. 86400 IN DS 47221 7 2 5A10389E282CB03F1083C1662B218B97CFE61024B991C7C5D8BB5685 AD6487CA
couldn't get address for 'a0.pro.afilias-nst.info': failure
couldn't get address for 'a2.pro.afilias-nst.info': failure
couldn't get address for 'b0.pro.afilias-nst.org': failure
couldn't get address for 'b2.pro.afilias-nst.org': failure
couldn't get address for 'c0.pro.afilias' -nst.info': failure
...
how to diagnose?