i can think of 2 things that can cause an issue.
1 = Bandwidth, a DP1.2 output has 21.6Gbps of bandwidth.
A screen at 2560×1600 60Hz in 16Bits color takes 13.27Gbps
A screen at 2560×1440 60Hz in 16Bits color takes already 11.94Gbps
OTgether that will be more thatn you have bandwidth available. My advise would be to lower the color depth to 12Bit than it should fit (19.61 Gbps in total).
http://www.kramerelectronics.com/support/bwcalculator.asp

2 Sometimes you need to activate DP1.2 in Dell Screens …

By default the Dell monitors are not enabled with this feature.

1. In the monitor’s on screen menu (OSD), navigate to “Other Settings”
2. Change the setting for “DisplayPort 1.2” to Enable
3. Confirm the selection

You may need to reconnect the DisplayPort cable after making this change.

Hope it helps …