Collision on coordinator when "End devices" sending simultaneously

I’m a newbie, I’m using 10 items of S2C modules ( Mode API: ATAP=2), 1 coordinator, 1 router, and 8 end devices. The problem is when the “gateway (coordinator)” recieve data from more than one “End devices” at the same time. In this situation (“End devices” sending simultaneously) the “gateway (coordinator)” only recieves one packet (and only uploads that data to the website). What is the best way in order to don’t lose any packet ?
I’m reading something about randomization transmission: “… each send transmission occur 3 times with a random delay from 400-1200 milliseconds between transmissions (used trial and error to determine those values)…” But it’s seem to be too perfectible. There is no handshake solution, ie all solutions to disable collision or obstruction on coordinator ?
What is your opinion, or best way ?

Could you please help in fixing the problem?
Any help would be appreciated.
Best regards.

The best way is to switch to a Polled method. This way only the node who is being Polled talks.

Dear MVUT,
thanks a lot.
Best regards.