ZED Join the network problem

hi

I use the chip is CC2530,the version of the protocol i use is Z-Stack 2.5.1a

I build a network, and the network include a ZC,a ZR and more than 30 ZEDs.

Firstly, the ZEDs can communicate with ZR or ZC smoothly. but once some ZED lost link to the ZC or ZR.

The ZEDs can not rejoin the network again unless initialize the ZC or ZR configration again.

After the ZEDs lost the connection to ZR or ZC,the ZED will send the Beacon Frame periodically. when the ZC and ZR 

receive the Beacon Frame from the ZEDs, they will give the ZEDs a response.

and i found the value of Assoc bit is 0 .

I think some table in ZC and ZR is overflow,and this time can not allow the ZEDs to join the network.

have somebody encounter this problem,

please give me some advices and explain why.

thank you! 

regards!