The rear wheels might have been placed on buggy in reverse. To test, look fora sequence of dots on the wheels (the brake cog), if you can see it from the side, the wheels have been put on reverse. The sequence of dots is the brake cog, where the brake wire slots into these holes to engage the brake (so the cogs should be facing inwards).
To fix, remove the axle from the wheel and insert the wheel the correct way. If the wheels are installed correctly and the brakes are still not working, please cease using the buggy and make immediate contact with your retailer or us directly.