You mean because it's jumping around on the radar? Lock the target. It should say Lock, just as you would with FLIR. With radar, it doesn't necessarily require this, though, and doesn't happen automatically.
The way the Longbow Hellfire is supposed to work, by the way, is in LOBL the Hellfire fires direct and locks on with the missile's radar prior to launch. LOAL is supposed to require radio updates until the terminal phase when the Hellfire's radar takes over. I'm not sure if EECH models these radio links or the necessity to update targeting information when the object moves.
Also, realize that right now a certain percentage of the time the EECH targeting system is off. Whether it's missiles or cannon, some targets appear to have a discrepancy between where the object appears to be (and apparently is in the sim) and where you and your wingmen appear to think the object is. This is why some tanks, for instance, will appear to survive ridiculous numbers of missiles and are still there.