XBee Errors with 15 motes test

We are facing a problem when we initiate a sync signal from a particular mote and then 15 other motes receives it, so when they receive it they go into random delay of 5 second, so each one of them goes into a function that picks a number between 0-5 second and then broadcasts a signal but when that happens we can see from the below picture that an error happens and we do not actually know what is the reason but when we decrease the number to 8 motes with 5 second of Random delay they broadcast with no error. please advice us what can we do in terms of XBee configuration or in terms of coding to avoid this problem.
it should be noted that this error make them shift in time as well as miss so many other packets.