KevNC...I haven't had my straps slip using the metal buckles. My nephews love to ride on the tramps...they are pre-teens but are 6 ft tall and weigh nearly
as much as I do. I am wondering if the type of nylon strap may make a difference. One kind may be more 'slippery' than another. I have had lashing straps slip like that...while others never move an inch. I guess I was lucky when I bought my straps.
So you could try to switch to another strap brand and see if those straps slip.
Another possibility is the amount of 'bite' on the teeth of the buckles. I know that some buckles are more 'aggressive' than others.