cacti graph not poll data in certain period time

[azharmg]

hi all,
pls help me that i faced some issue in graf mrtg (cacti)
in cacti all graph failed to poll data in same period...flat graf...
BUT in that server also have mrtg running backgroud, the graph is fine (have traffic there)

what is the issue here and pls advise me on how to tune the cacti settings.

Thanks