perhaps you could try landing split foot (that is, your feet aren't parallel with each other) and slowly work from there. split foot --> one-two landing, etc. --> running landing. could also be that you're slightly leaning back when you land so that you're actually reaching forward with your legs (might subconsciously happen when you're trying really hard to clear an obstacle or get distance afterward); if that's the case, then stick to an obstacle you're comfortable with and keep your legs more or less directly below your center of mass... should help a bit. keep in mind, by landing with one foot first, you're increasing the impact taken by that one foot, so be careful. happy training =)