I found this problem since Op7, and it still happen in Op8(8050)

I always found some monitor stop for no reason. For example, sometimes i check my switch found that one interface have no flow data since 3 days ago, i try to "update status", but nothing happened. Some monitor i can make a test immediately by myself, but i can't do it on interface monitor. If i successfully make a test by myself and get a monitor data, after that the monitor will work again.

Please help me. I make flow report to my boss every week, there always some port stopped for no reason, and i have no idea. I tried SolarWinds for some time, it works fine, so i don't think it is my problem(or my switches).