Fix A Flat does work on some slow tire leaks.
Fix A Flat cannot fix all leaks but a slow leak the Fix A Flat should be able to seal the leak just fine.
But if the slow leak is in the bead of the tire which is where the tire makes contact with the rim to seal it then the fix a flat cannot seal that bead leak.
For a slow leak to be fixed in the bead of the tire the tire needs to be removed from the rim and the rim needs to be cleaned up and the bead of the tire needs cleaned as well.
Then if the rim is in good enough condition then the tire should be able to be resealed again on the rim.
Fix A Flat works best on small nail holes and if you have a large nail hole then patching or plugging the tire is your best option.
If you do have a nail or screw or other metal object in your tire tread and it's slowly leaking then leave the nail or other thing in the tire and put the fix a flat in to help seal it.
Then go to a tire shop and have the tire shop plug or patch the tire.