I too have had the same problem. I have a brand new PA14 and the first time I thought it was because the cross bar weld had broken. I have a 50 yard trek down a gravel path from my truck to the water. At the end of a long day, I was exhausted and didn't think much of the resistance except that I was beat. Realized when I got to level ground that something was not right. I got a replacement cart from my local dealer, but it only took 25 ft this weekend to realize that the collars were actually slipping due to the weight of the kayak. Lost a whole day fishing and had to get help to carry the kayak back to my truck.
I came up with the PVC fix too, but I shouldn't have had too. Especially after spending $160 on a cart that was not designed correctly. Hobie should have figured this out before the customers did! Now I have a "rigged" cart and two worn spots on my scuppers from the tire rubbing that has me concerned about the integrity of my new boat! Hobie's getting an earful and some pictures.