When you export a map, you need to make sure when you go into the map through map maker that you have chosen to make the portal appear:
Once you have loaded your map in map maker, there are four tabs on the left correct? For water, sizes, objects, etc. Well, on the first tab, there should be, roughly in the center, a little box with the writing next to it: 'Has Portal' And several empty boxes beneath. You need to make sure there is a little white tick in the box by the writing, just click on it to tick it.
If this is useless I hope you manage to fix it soon.
