But the new radwagon still says the carry load for the back is 120lbs. Why can other brands carry so much more on the back? The tern can carry 400lbs between the front and back. Whats up with the radwagon that it can't do it without the spokes crackling?

Hi there!
I love my radwagon, but my daughter is getting heavier and to carry her and one of her friends, it's over 120lbs. I could get a pricey yuba, etc, so that I could get more weight on the back without the spokeds doing that clicking thing.... but I don't want to. What can I do? Could I get a higher quality rim and spokes. Would this help? I'd like to be able to carry up to 180-200lbs on the back. Any suggestions? I've tightened the spokes in the past and it helps for 50-100 miles and then it's back to clicking even with just 80lbs on the back.
Got advice for me?
Thanks so much!