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We have a 6-month old son and when he sits in the BOB, sure enough, he slouches. The problem is that the seat back is made of a pretty soft material. The red strap that is supposed to support the mid back is up near his shoulders. Hence, his back sags into the seat causing the slouch.
The fix is to place another strap around the lower back of the seat to provide a lumbar support. See the first photo. I added the black strap that goes below the red strap. You can buy any luggage strap that's long enough.
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Try it out and let me know how if you find this fix helpful.