Data delay on XBEE S2 802.15.4 Network


I had a configured Xbee network (802.15.4) with 7 modules (being one the coordinator) and all others as end devices (all modules connected to Arduinos). Applied to a Wireless sensor network, however I am having some issues with a delay between the messages, all messages are fully send/receive but sometimes the xbee network delay on delivery the messages (holding it for a few aleatory seconds between 1s and 7s that I dont know why!). My modules send continuously messages that some times it arrive separately (it is right, is what should happening) and sometimes they came together (caused by the delay). Im monitoring the messages (strings) with several terminal commands, that is how I discovery this issue.
Has some one any idea of why is it happening?

I think if you use default settings on all the modules the performance will be better.
Also check the Sleep parameters on the end devices.