[Norfleet]

It has to do with the terrain: Swamps, forests, and mountains will all halt the movement of any non-fliers, unless the entire army has the relevant survival ability. Wastes, plains, and farms allow full use of > 1 strat move. An enemy-held province at the endpoint of your intended move will also prevent a non-flying unit from performing multi-move into that province.

Ocean squares seem to never allow more than one move, with the exception of units that sail, and can skip over one sea square to land on any land square.