I believe it has a bug - but to get it to load what I had to do was to place it where it needed to be but rotated backwards, and then use R to rotate it in place to be the right orientation. Unloading though works properly but only if you place it correctly the first time, if you rotate it into place then it stops unloading.
I have to think this is a bug - as it stands I can't find any way to have the same loader unload from an inventory and load into another inventory (a single loader) - because you have to place it wrong first for the loading to work properly but you have to place it correctly first for the unloading to work properly.