There are lots of reasons why birds wont fly, I have an open position and birds cant land anywhere but on the loft, so the old birds on natural were flagged twice a day for an hour and worked perfectly.
Now my youngsters are a different matter if they are healthy they will fly and fly mine did up to 2 hours some days, I say some as they don't always do it, but I like birds to come out of the loft and up and away doesn't matter how long the fly as long as they want to go up from the moment the door is opened.
I never hold any interest in ranging either they do or they don't, it makes no difference in my opinion to quality, health or homing ability. I have had years I couldn't get the youngsters out of the sky they would almost fly all day...worst racing season I ever had hopeless. Then other years they don't fly too much but ok yet raced like their lives depended on it, but always healthy.
I would say if your birds are really reluctant to fly then your either feeding too heavy, put them on barley for a week, or they need medication for worms or canker, if you chaise them off the loft and when they come back from a fly they are panting and its nota hot day then maybe its respitory.