As mr froggy asked above,did you check that all sprite and enemy properties are correct?
Some things to check: position of the sprites in the image and of the enemy in the map, that the enemy and sprites exist, the sprites' origin point, etc. Make sure that you start an animation and that it does exist. Remember that the engine in many cases starts automatically the walking animation on enemies, and if that one does not exist you may see nothing at all (it is easy to override this built-in behavior using sprite/entity events, as I do with my enemies).