MacBook Pro Retina & Dell U2711 with miniDP to DP still no res higher than 1080p I bought an external monitor, the Dell u2711 due to the res and wanting to get the most out of my laptop.

Initially I used a mini dp adapter with dvi, hdmi and dp on and it wouldn't work, kept staying in power saver mode so I bought a single cable with minidp on one end and dp on the other.  Plugged it in and voila, it works.  Then I checked the resolution and have two options.  720p or 1080p.  I had more options using HDMI but I want 2560x1440.  That's why I bought it.  Can anyone help please?
Added as requested: the solution was to hard rest the monitor by pulling the power and them power cycling the laptop. All is perfect now:)