Missing flight information from live flight tracker


I live in Great Neck, NY (by Middle Neck Road and Piccadilly Road), and I’m using live flight tracker to keep track of flights flying near my home.
However, I found that there are some planes flying by, but not shown on the live tracker.

For example, I notices at least 5 planes flying on top of my house in very low altitude during 7:05PM - 7:20PM EST 02/11/2015, but I only saw at most two of them on the live light tracker.
I tried another website (webtrak5.bksv.com/panynj4), and didn’t find those flight either.

Those flights were in very low altitude, and the flight trajectory was almost the same.
If possible, could anyone share some feedback on why they were not shown in the tracker? Thanks!
I wonder if it’s related to NextGen experiment by LGA.

Not all flights are displayed. Read the FAQ

If you want to see most (not all ) of what is going overhead, build yourself a ground station and monitor directly. This won’t catch everything- GA flying VFR don’t announce positions, nor do some mil. For groundstations, see

I see, thanks a lot for the prompt reply!