I have a solid invisible object that when the character collides with it the character the movement stops (start moving in direction [center square]). However, this doesn't happen. Why?

Is the collision event on the character or the invisible object?
If it is in the block then you apply the action to the OTHER instance if it is in the character then you apply it to SELF.

I have tried it both ways, it just doesn't work.

Even if an object is invisible it needs a sprite for it to have a collision mask.Have you asigned a sprite to your invisible object?Also if your character is moving to fast it may go trough the object,how fast is it going?
Also,sometimes we make objects and forget to put them in the room.