#1 fping gets response for only 2 first reqs, ping works fine

[wiku]

I use SmokePing and fping as a probe. fping version is fping: Version 2.2b1 $Date: 1997/01/08 20:29:33 $
fping: comments to fping@networking.Stanford.EDU

but I've checked that the same problem concerns
fping: Version 2.4b2_to_ipv6 $Date: 2002/01/16 00:33:42 $
fping: comments to david@remote.net

For all hosts it works fine except okimpan-gw.man.poznan.pl(150.254.240.81). This one responses for only 2 first requests, even if there are running two fpings in parallel:
weed> fping -C 5 okimpan-gw.man.poznan.pl
okimpan-gw : [0], 84 bytes, 2.02 ms (2.02 avg, 0% loss)
okimpan-gw : [1], 84 bytes, 5.79 ms (3.90 avg, 0% loss)

okimpan-gw : 20.2 57.9 - - -

while:
weed> ping -c 5 okimpan-gw.man.poznan.pl
PING okimpan-gw.man.poznan.pl (150.254.240.81) 56(84) bytes of data.
64 bytes from okimpan-gw.man.poznan.pl (150.254.240.81): icmp_seq=1 ttl=61 time=2.31 ms
64 bytes from okimpan-gw.man.poznan.pl (150.254.240.81): icmp_seq=2 ttl=61 time=2.56 ms
64 bytes from okimpan-gw.man.poznan.pl (150.254.240.81): icmp_seq=3 ttl=61 time=2.42 ms
64 bytes from okimpan-gw.man.poznan.pl (150.254.240.81): icmp_seq=4 ttl=61 time=2.29 ms
64 bytes from okimpan-gw.man.poznan.pl (150.254.240.81): icmp_seq=5 ttl=61 time=2.16 ms

--- okimpan-gw.man.poznan.pl ping statistics ---
5 packets transmitted, 5 received, 0% packet loss, time 4039ms
rtt min/avg/max/mdev = 2.166/2.354/2.564/0.140 ms

The interval, for which host okimpan-gw responses for all requests is >= 4000 ms

As far as I know there is no firewall on okimpan-gw or between my host and okimpan-gw. From administrator of okimpan-gw (unfortunately it is not me) okimpan-gw is hardware router Linksys RV082.