Many of the X's are probably due to entering misorders (for the supports) or that one of the supports was cut, or the move failed. A support is cut simply when an enemy unit attacks the province. So lets say you're defending Venice from Austria. Austria has units in Trieste and Adriatic Sea. You have units in Venice and Piedmont. If Piedmont support holds Venice, then Austria will never be able to take it.
But, if Austria moves a unit to Tyrolia OR France moves on Piedmont, you won't be able to hold it.
In many situations, the tactics of which units support whom where is critical to success. You must anticipate the enemy's move and counter it.
Let's say you want to take Rumania (as Turkey). You ended up with units in Bulgaria, Serbia, and Black Sea. Russia has armies in Sev and Rumania, and a fleet in Armenia. Let's say for our purposes Austria is dead and Italy has his home SC's and is your ally, so won't attack you. Now, even though it makes more sense to take Rumania with an army than with a fleet, you shouldn't do it. Why? Rumania has one support hold. You need two to take it. But you should notice that Armenia will likely tap Black Sea (again, assumption is that Ankara is covered and A Moscow will move to Sev). So instead of having Black Sea support Bulgaria there and have the move fail due to lack of supports, you anticipate which support will be cut, and move that unit over the more ideal one.