Why can't I get the right screen resolution in Ubuntu?

Due to buggy hardware or drivers, your monitor's correct resolutions may not always be detected. For example, the EDID data block queried from your monitor may be incorrect.

If the mode already exists, but just isn't associated for the particular output, you can add it like this:

$ xrandr --addmode S-video 800x600

If the mode doesn't yet exist, you'll need to create it first by specifying a modeline:

$ xrandr --newmode

You may create a modeline using the gtf or cvt utility. For example, if you want to add a mode with resolution 800x600, you can enter the following command: (The output is shown following.)

$ cvt 800 600 # 800x600 59.86 Hz (CVT 0.48M3) hsync: 37.35 kHz; pclk: 38.25 MHz Modeline "800x600_60.00" 38.25 800 832 912 1024 600 603 607 624 -hsync +vsync

Then copy the information after the word "Modeline" into the xrandr command:

$ xrandr --newmode "800x600_60.00"
38.25 800 832 912 1024 600 603 607 624 -hsync +vsync

After the mode is entered, it needs to be added to the output using the --addmode command as explained above.

Source: https://wiki.ubuntu.com/X/Config/Resolution

I just ran into same problem and solved by following steps:

1.. Find configuration for desired resolution and add it to xrandr:

$ gtf 1920 1080 60
 
  # 1920x1080 @ 60.00 Hz (GTF) hsync: 67.08 kHz; pclk: 172.80 MHz
  Modeline "1920x1080_60.00"  172.80  1920 2040 2248 2576  1080 1081 1084 1118  -HSync +Vsync
 
$ xrandr --newmode "1920x1080" 172.80  1920 2040 2248 2576  1080 1081 1084 1118  -HSync +Vsync
$ xrandr --addmode VGA1 1920x1080

2.. then this new mode can be used:

$ xrandr --output VGA1 --mode 1920x1080