I'm currently having trouble determining why some provinces allow me amphibious assautls and some dont. Im playing England, and have ports in along my coastline and have ships all along the mediteranean coast in an uninterrupted line.
Now, I if take a stack and hold it up to see which provinces turn blue, allowing me to move or attack, many (in fact, all) provinces near my territory are available. However, the provinces along the far Med (egypt, tripoli, most of Italy, Byzantine) are not available. More confusing, some of them are available and it isnt apparent why, such as Spanish held africa and one(?) province of the entire boot of Italy.

Now, Ive checked and there are no enemy ships breaking the trade route. The theory that I can only invade enemy provinces and not neutral s or allies doesnt hold up because a province in allied italy is allowed.. when the one right next to it isnt, from the same faction. The theory that the distance is too great doesnt work because I can attack faraway (spanish held) africa but not egypt right beside it?

What determines whether I can move my troops by sea? Im boxed in by land by alliances and it seems it's my only viable alternative for offensive action..