I think if you send them to scout a specific country and they end their mission in a few days it's because they did their job!
I will tell you what i do and everything is in order.
First thing to do is to remove all scout assignments except the "Scout League" and "Next Opposition".For these two you must have two good scouts who know the country you play very well.
Then,i buy one scout for every region(cause my team doesn't have good scouts):
1) Central Europe (French or German)
2) South Europe (Italian,Spanish or Portuguese)
3) Eastern Europe (Serbian,Russian or Ukranian)
4) UK and Ireland (Brittish)
5) Scandinavia (Swedish,Finnish,Norwegian or Danish)
6) South America (Brazilian or Argentinian)
7) North Africa (someone African-Finidi George is a good addition)
After that, i remove their automatically added scout assignments and i add a new assignment->region."Roaming" is always ticked.
Do that and you will do fine.Let me know if everything is in order dude.