There are several possibilities that could cause this. What comes right to my mind is that maybe the software version on the RE1 is somehow different from RE0. This is less likely on M120 compared to some older models, I suppose, but sometimes if the version is different enough the Master RE cannot connect to the backup.
Another thing to mention that shouldn't happen in M120 but is common in other models such as M320 or T640 is to have the Routing-Engine installed without the corresponding control-board or MCS (for M160). These routers cannot power on the routing-engine without the corresponding control-board to form a complete host module. I mention this here just for reference in case others browse this topic.
I find that the best way to troubleshoot RE issues is through the console. Can you log into RE1 through the console port and ensure that it is functioning? Also, what does "show chassis hardware" say about the two RE's?