Solr nodes' replication is getting stuck

We have standalone solr servers which are master and slave. Also have a full indexer job nightly. Generally, when job executed successful everything is alright. But last days, we noticed that indexer node has different document number with searching node. So, expected productions are not available in our production system. That's why we had to restart nodes and start replication manually, then problem went away. We need to prevent to occur this problem again. What do you suggest us to check or where should i look at? Indeed i think that essential error about the issue is: "SEVERE: No files to download for index generation" Regards