This has happened to us many times. I believe that, over time, with the flexing of the leading edge spars during flight, the glue used in production finds it's way loose. When I find that condition I remove the ferrule, carefully scrape off the old glue and use super glue to re-install the ferrule. I wrap masking tape around the ferrule at the point where you want the ferrule to stop sliding into the spar, as once you apply the glue to the ferrule and start sliding it into the spar, it should be done quickly. The tape provides a "stop" so that the glue does not set up before the ferrule is "home", or you don't slide the ferrule too far in and the glue set before you can pull back out to the proper place. Just a tip.
Lynn and I do a lot of flying. When I am washing our kites, or otherwise " messing" with them, I always pull the center spar out and do my best to try to twist the ferrules out of the center spar, thus avoiding the chance of pushing a ferrule all the way into the spar during assembly, which is a major pain, or pushing it in all but a half inch or so, allowing assembly, but after a short time it will splinter the end of the outer spar. The latter happened to one of our team members just yesterday.
Hope this helps.