Part of them being reluctant to fly much could just be that they aren't confident enough too do crazy fly times yet. If they haven't been flighted for very long (or they haven't flown much), they essentially have to learn everything about flight; taking off, manoeuvring midair, guaging placement of objects while they're flying, how fast they can/should fly, finding appropriate places to land, guaging landings and actually landing, etc. It can take a while!
Chewy has been flighted for over two years and she is nowhere close to a pro at flying. She used to learn how to land in one spot at a time (ie- when she was first learning to fly, she would only land on top of her PVC gym. Then it was a lower perch on the gym. Etc.), but she is now just starting to land in spots she never has in the past. This evening she couldn't find a good flightpath from her playstand to me, so she flew and landed on top of my laptop screen instead. That was a first, and such a big step for her!
I found with her, recall training was the key to encouraging flight. She always wants to be with me! But it took weeks of training to even get her to fly a very short distance to my arm, that's how unconfident she was. She has come a long way and will now recall across a whole room, as long as she feels comfortable in that room and there are no distractors. Some birds just take longer to learn flight than others!