So this setup depends on what kind of laptop you have attached. What model of system is the WD19 attached to? Refer to the display resolution table in the user guide:

If your laptop supports HBR3 lanes, then you should be able to run 2 monitors at 2560 x 1440 @60 Hz. If your laptop only has HBR2 lanes, then you will be limited to 1920x1080 @60 Hz. Note that both lane types have a limit of 60 Hz for the refresh type. I would try reducing the refresh rate of the 1440p display to see if you get the full resolution, but that will have to be a compromise of the refresh rate. 

Yeah, this is almost certainly a bandwidth limit rather than a faulty monitor, since the 27-inch panel works fine on its own. Try setting that 27-inch display to 2560 x 1440 at 60 Hz first, because asking it for 140 Hz at the same time as a second screen leaves the dock short on bandwidth. In Windows, do that under Display settings > Advanced display, then unplug the dock cable for a few seconds and plug it back in so it renegotiates both screens. If it still drops to 1080p, check whether your laptop port is actually Thunderbolt or just USB-C with DisplayPort, since a plain USB-C port will be tighter on bandwidth. Let us know the laptop model and which dock ports each monitor is using, as that will narrow down whether HBR2 or HBR3 is the ceiling here.

To add to the excellent replies already here:

The WD19 is available in multiple formats.  The WD19 and WD19S are USB docks -- only the WD19TB(S) has a Thunderbolt port.  Even if your notebook has a Thunderbolt port, with a WD19 or WD19S model, you're limited in bandwidth to the docking station.

Hey, thank you for your replies.

The problem is that the WD19S doesn't support Thunderbolt and is limited to two monitors, both running at 1920 × 1080 @ 60 Hz.

I'm currently using it with a Dell Inspiron that has Thunderbolt 3.