Can't Assign Army to Front Lines

[Carbon14]

I am very new to this game but have watched beginner guides on the basics. Currently I am just trying to put my army on the front lines with another country. However whenever I assign them the order they don't follow. I may be doing something wrong in which case just correct me.
I clicked the front line button to create a new front line
Then left clicked on another country's border to create front line which assigned them to it
Then clicked the execute button for the army.
I probably did something wrong but I have no idea. I got it to work once but not a second time.

[runifoc]

The front line needs to be on your side of the border unless you're already at war with them. Did you drag the cursor along the provinces on the border so you saw the front line icons appear?