Thinking back,, I guess Ive been fortunate in that all the dogs I have had, liked to carry objects. It started with "toys" when they were really young. (Weeks) I really encourged this and praised when they did it..
Once on a leash, and walking with me, I would take objects with us, and encourage them to carry those, eventually transferring over to a canvas puppy bumper..
So, I dont remember a dog stuggling with carrying at heel when we got to HOLD.. but, I didn't look at it with a very critical eye.. If the dog dropped it,, I just picked it up, and put it back in the dogs mouth, repeating this till I believed the dog had a good general idea what I wanted.. I didn't expect perfection..
Once Obedience commands were solid, especially "sit",, and the dog would "Hold" reasonably well,,and walk a short distance with a bumper in mouth,, we moved on to FF
I Believe now,,, you have to pick your battles... IMHO,,, if the dog will open its mouth,and hold a bumper for a period of time,but may struggle with holding it , after a command of sit,, or walking a ways,, I would pick my battle..
I still would make sure the dog "sit" is solid before you start FF.. Just makes things way easier, than having to wrestle the dog.. That additional activity only spins both dog and trainer up making for a more stressful atmosphere than FF already MAY be..