I see failing check for localhost and one extra host I added.
1. How can I examine check output to find failure reason?
2. Is it possible to see it via webui ?
I tried to raise debug level to INFO / DEBUG in /etc/shinken/pollerd.ini but and restarting, but still check result seems be not included in logs.
3. Is there any other way to find why checks are failing?
Its commands/check_host_alive.cfg test:
command_line    $NAGIOSPLUGINSDIR$/check_ping -H $HOSTADDRESS$ -w 1000,100% -c 3000,100% -p 1
which runs without error from shell:

[shinken@monitoring2 /etc/shinken]$ /usr/lib/nagios/plugins/check_ping -H localhost  -w 1000,100% -c 3000,100% -p 1                                                                                                                                                   
PING OK - Packet loss = 0%, RTA = 0.04 ms|rta=0.043000ms;1000.000000;3000.000000;0.000000 pl=0%;100;100;0