fsa comet coming loose

kirby700

My bike was in getting some work done and i know they removed the cranks. There was no problem with this. I rode the bike the next week and at about 22miles out the cranks could be felt that they're loose. I stopped and loosened the crank bolts and and tightened the preload bolt and did the arms up tight. All good and thought apart from they came loose again after about 18miles. Is there anything i am doing that's wrong or do i need to go back to halfords.
Cheers