I have found the same thing, the plot route feature does not work if the distance is too far. However if you step towards the destination gradually sometimes it 'loads' the route better and allows you to plot the destination.
My technique for plotting a destination is as follows:
Open galaxy map
Click on navigation tab
Type the name of the destination you want to reach, press enter and this will zoom you to the destination location
With mouse click on this system to select destination, or if available click plot route
If you can't plot a route that far, then what I do is.
Click the current location > arrow, this will zoom you back to where you are currently
Then rotate the camera so that I can see my current destination with a blue icon, and forward ahead of me into the distance I can see the travel destination I want to see with an orange icon
Now that is the general direction you want to travel, depending on the range of your frame shift drive and how many nearby system there are this will determine if you can actually plot a route to where you want to go
What you can do is start clicking on the systems in the direction of your travel destination, this shifts the camera focus like taking small steps towards the destination.
Keep stepping through the systems by clicking each one until the 'plot route' option becomes red. This will be as far as you can travel. Go back a step, plot route. Then begin your travel and make all the jumps out to this point.
Generally you should be able to travel out to this point and then use the same method to plot the next path of the journey. Basically for some reason the route plots only work out to a certain distance. I find it's around 100LY, further than that it doesnt allow me to plot a route. I think this improves as your FSJ improves.