I setup a Nginx server as a proxy on front-end, and configure two Apache server behind it, with the upstream configuration, it works fine.
Currently, according to the loading increased, I add a new Apache server into the group, but once I active the new Apache server, all loading will be pushed to the new Apache server, the original two servers are free, below is the upstream setting of mine.

upstream apache_ads {
server ads-del.s2.zoomino.cn:9080 weight=1;
server ads-del.s1.zoomino.cn:9080 weight=1;
#server ads-del.s5.zoomino.cn:9080 weight=1;
}

As you see, server ads-del.s5.zoomino.cn is the new one, ads-del.s2.zoomino.cn and ads-del.s1.zoomino.cn is the original ones, my Nginx server is running on server ads-del.s2.zoomino.cn, but it listens port 80.

I want to know, why are most of the load pushed to new server, but not share normally with three servers? Thanks.