I have a Precision 7520 sitting on a Pro2x Port Replicator.  I have two Dell 24" S2721HN monitors plugged into the two DisplayPort ports on the replicator, from HDMI port on the monitor.  Each monitor has a resolution set to 1920x1080 60hz.

Both monitors were recognized and worked fine for about a year.  Then a couple of months ago, one of them stopped working, and is no longer recognized when plugged into the port replicator.  I have tried a new cable, and a new replicator, but neither worked.  I can use an HDMI-to-HDMI cable to plug the 2nd monitor directly to the laptop, and that does work.  I can now use both, but I'm wondering what may have happened to make the 2nd display port stop working on the port replicator.