Having a problem towing a car on a trailer?

Due to trailer size in relation to vehicle size I had to use a highly modified tie down arrangement and never had the vehicle move. The front had slings run through the lower control arms with a single tie down run through the slings to the two outer D-rings. This arrangement gave me about 2" of deflection in the tie down Under certain situations you would expect the front of the vehicle to move from side to side. The rear had slings around the trailing arms with 2 tie downs that crossed as they ran to the rear outer tie down D-rings. This angle was also very tight, the straps touched the rear tires. In 18 months and 36,000 miles I never had to retighten my tie downs (3") and this tie down arrangement should have been very prone to movement. My guess is that your tie downs are not as good as you believe and are stretching or you are just not cranking them down enough. Another recommendation is to hook your ratchet end to the trailer tie down, run the strap to the vehicle and then back to the same trailer point or hook it to the ratchet hook if the trailer tie down point is to clumsy to reach. This double up allows the tie down to move slightly in relation to the vehicle. We always tied the race cars down using the double up technique and they never moved.
Randy