mikrotik is invisible in nagios ( no icmp echo)

Hello,
I've got a small (20 APs running on Mikrotik routerOS 2.9 on 5.4Ghz)
network .
I'm monitoring it with nagios..
My problem is that when i try to ping mikrotik machines by normal ping
- they does not respond to it.. when i ping it with arping - it
responds and then for a few minutes i can ping them with icmp ping...
after few minutes (even seconds ) it stops responding to icmp...
what is the problem?

(theese MTiks are configured to bridge wlan with ethernet and the IP
address that I ping is ethernet ) configured like this :

[admin@MTD] > ip address print
Flags: X - disabled, I - invalid, D - dynamic
# ADDRESS NETWORK BROADCAST INTERFACE
0 10.100.3.100/24 10.100.3.0 10.100.3.255 ether1

I ping it from 10.100.0.1 that has an ip alias too for Mikrotik's net
( that is 10.100.3.1)..

please help
regards
WZ