Altcoin peers do not sync with each other I created two altcoin peers and they can add blocks to genesis block. However their blockchains do not sync with each other!

The only way let the block merge is to stop a peer and restart the peer. 

Is there any solution? Thanks..
Solution found! 
Bitcoin client use checkpoints to prevent 51% attack. What are checkpoints?

Modifying checkpoints in chainparams.cpp solve this sync problem.