Looks like you may have mixed up the ‘closing’ straps / buckles with the saddle ones - http://road.cc/content/review/182156-apidura-saddle-pack-mid-size
Yes, YES! That's exactly what I've done! Thanks for solving the mystery. It was so bad it was making me seasick! Climbing out of the saddle was horrid!
It also helps to make sure the ‘nose’ is really tightly packed. If you have any space in there it loosens up over time.
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Looks like you may have mixed up the ‘closing’ straps / buckles with the saddle ones - http://road.cc/content/review/182156-apidura-saddle-pack-mid-size