I just connected my external monitor and it all works fine. I tested this even in windows. But on ubuntu I cannot take the cursor to the external monitor. It just doesn't move there and it gets stuck at the laptop screen boundary. I cannot move it beyond that.
Most likely one of these situations is the case (In the example, HP 22" is my main screen)
The screen is misaligned or in another position than you assume
Open System settings > Screens:
1. the screen is misaligned
2. the screen is positioned differently from what you assume: on the left side..
... or on top:
Simply drag the window in another position if one of these situations is the case.
Another thing that makes it easier to move from one screen to another is switching the "Sticky edges" ("plakranden" in the screen shot below) option to off:
Looks in System Settings - > Displays how configuration of your system looks like. If all is okay and it looks like you predicted then try to move your mouse fast, sometimes my screen may not work for some reasons even if I know that I'm on the level of the screen I need. If all of it isn't work problem may be with the cabal change your hdmi/dvi or other.
Another possibility (which is rare) is there are some space between monitors. This means, even though left and right monitor are aligned with each other, there are some tiny space between these two. In my case, it is about 2.5mm visible space on my 24"in wide screen. So it feels like hard to move between boundaries. What actually happens is that cursor is moving in those ghost space area you can't see and therefore cause the delay.
The fix is to make sure monitor are really side by side on your 'display' setting.