If you can post your code we can take a look, place it in a .zip file and upload it.
I did this about a month ago, had some problems because with an array inside an AOI (aoi_ARRAY[x])which the FFL loaded values too needed to have another array tag outside the AOI to serve as a reference (ref_ARRAY[x]) this is due to the Nature of the InOut parameter type required to be used for an array.
When you assign the reference array in the AOI instruction call you just use the ARRAY tag name only, no need for brackets.
Ref_ARRAY --> aoi_ARRAY not Ref_ARRAY[0] --> aoi_ARRAY[0]
You aren't assigning a length parameter like a COP instruction.
Of course, the ref_ARRAY size should be the same as the aoi_ARRAY size. Also be sure to put a lot of logic in the AOI to verify array size, FFL .POS, last thing you want is to fault your processor because your array size was off and the FFL is trying to reference an invalid array element.
FFL/FFU logic and be problematic if you aren't used to them at first, be sure it's bullet-proof before you place it into an AOI and use it within a production environment.