I have a Sony SVS13112FXB laptop. I have been uning it with a Samsung SA350 monitor. Two other computers are connected to the monitor through an IOGear switch. I get full screen resolution with all of them when connected to monitor through the switch.
I recently added a VGPPRS30 port replicator for the above-mentioned laptop. While everything works (I am typing this item on this machine) the maximum screen resolution is now 1024x768 (4:3) and no longer does Ubuntu discover the monitor.
Reconnecting the video connection directly to the laptop restores full resolution immediately. Not surprisingly Sony has been of no help as I have removed the original OS from the system in order to use Linux.

Hello,
I suspect it could happen because your displays are DDC dependent ("modern" displays give their specs to the pc through a special data channel) and the port replicator prevents the displays from communicating with the computer. Ubuntu can thus not know the screens resolutions.
Try adding the mode through xrandr manually
where I assume your optimal mode is 1920x1080, and your display is VGA1 (type xrandr with no argument to list detected displays).
If that still doesn't work, you may try specifying the modeline yourself. For that, get the modeline from the following command
and use that information for the following command
If that works, you may read the following documentation, section "statically setup in xorg.conf", to make the change permanent :
http://https://wiki.ubuntu.com/X/Config/Resolution

I think your analysis as to the issue is likely correct. However, the solution you suggest, which all seems logical, had no impact.

Aw, if your use of xrandr was correct, I'm afraid I won't have any further ideas...
What is the output of xrandr when your screens are connected to the port replicator ?

Here is the full record:
xrandr -- addmode VGA1 1920x1080
usage: xrandr [options]

So that obviously did not work. Then:
cvt 1920 1080 60
# 1920x1080 59.96 Hz (CVT 2.07M9) hsync: 67.16 kHz; pclk: 173.00 MHz
Modeline "1920x1080_60.00" 173.00 1920 2048 2248 2576 1080 1083 1088 1120 -hsync +vsync

This is completely in line with what you had indicated. Finally:


xrandr --newmode "1920x1080_60.00" 173.00 1920 2048 2248 2576 1080 1083 1088 1120 -hsync +vsync
X Error of failed request: BadName (named color or font does not exist)
Major opcode of failed request: 150 (RANDR)
Minor opcode of failed request: 16 (RRCreateMode)
Serial number of failed request: 36
Current serial number in output stream: 36

This is not what I had gotten yesterday when I tried your suggestions, but that response is no longer available.

The error for xrandr --newmode might be because the mode is already known.
what does the xrandr command without arguments tells you ?

I'm glad you understand this. Here goes:
xrandr
Screen 0: minimum 320 x 200, current 1024 x 768, maximum 8192 x 8192
LVDS1 connected (normal left inverted right x axis y axis)
1366x768 60.0 +
1360x768 59.8 60.0
1024x768 60.0
800x600 60.3 56.2
640x480 59.9
VGA1 disconnected (normal left inverted right x axis y axis)
HDMI1 disconnected (normal left inverted right x axis y axis)
DP1 disconnected (normal left inverted right x axis y axis)
HDMI2 disconnected (normal left inverted right x axis y axis)
DP2 connected 1024x768+0+0 (normal left inverted right x axis y axis) 0mm x 0mm
1024x768 60.0*
800x600 60.3 56.2
848x480 60.0
640x480 59.9
1920x1080_60.00 (0xca) 173.0MHz
h: width 1920 start 2048 end 2248 total 2576 skew 0 clock 67.2KHz
v: height 1080 start 1083 end 1088 total 1120 clock 60.0Hz

Okay, your screen is actually attached to DP2 (Display Port 2). It seems the 1920x1080_60.00 mode is already added, so try this
You might try beforehand (if it doesn't work) :
or
Sometimes the screen actually needs to be set off and on again for those changes to work

Voila!

Here is what happened:
bruce@GSS-Sony:~$ xrandr --output DP2 --mode 1920x1080_60.00
xrandr: cannot find mode 1920x1080_60.00
bruce@GSS-Sony:~$ xrandr --addmode DP2 1920x1080
xrandr: cannot find mode "1920x1080"
bruce@GSS-Sony:~$ xrandr --addmode DP2 1920x1080_60.00
bruce@GSS-Sony:~$ xrandr --output DP2 --mode 1920x1080_60.00
The last line gave me the resolution desired. Thanks much.

Ok cool
That's the last two lines that were important, the 1920x1080_60.00 was defined, but not added to the DP2 display. That's what the second last command did. And then you've set the DP2 display to use that new mode.

I'm fairly computer literate. I was a software engineer for Intel for many years (the software product Math Kernel Library was my creation) and still do some consulting in my dotage. But Linux is so rich that it is often difficult to even know what to look for. Man pages are great, but some of the functions are extremely complex, including, I think, xrandr. On the other hand, the Linux community can be most helpful, as you have so ably demonstrated.