about ncp role for beaglebone black platform

i have three devices one end device(mtd), one ftd, and one ncp with beaglebone platform. If ncp become leader, i can’t ping google server from MTD device and FTD device(which is router). i am sure ncp and MTD,FTD in one thread network. if ncp become router, i can ping google server from MTD and FTD. Someone has same experience or not? thanks.
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Second question is: i still have three nodes, one mtd, one ftd, one ncp. when i reset each board, mtd become child role, ftd and ncp often easily become leaders role. so they are two partition. Even if i am waiting several minutes, they are still two leaders. why they can’t become one leader automatically when the radio is near? Thanks.