I’m not sure how long it has been now, but a little while ago roblox changed their system for pending for full games. It used to be that the little dialog box would sit on top of your browser, stating that its waiting for an opening. While it pended, you could be playing in another game server.
But now, instead, if you already have an instance of roblox running, and you attempt to pend for a full server, it replaces your current instance of roblox with the server you just requested, and waits for an opening.
I find this extremely frustrating, because often times there may be a specific server of a game I wish to join (for different reasons, perhaps to play with a friend, or to join a raid/defense at a clan base), but I could at least play in another server while i waited to get into the server that I wanted to join. This is no longer possible. (Though, I heard from a friend, apparently you can still do this on Mac)
As a gamer that enjoys playing with friends, i struggle with this issue multiple times a day. Although there is probably a valid reason as of why this feature got removed, i really hope they’ll bring this feature back.
Master3395 is correct - Chrome changed something a while back that made it so that ROBLOX cannot do this on Chrome. If you play on Firefox or IE, you can still do this.
The solution, then, is to use Firefox to pend for servers while you use Chrome to visit other places/servers.