I use a different brand for my 'x-chocks'. They loosen considerably once the tires have cooled after pulling in to camp and have to be re adjusted usually within 30 minutes, or less. From your description, it sounds like yours are loosening a lot more than mine.. but in any event, everyone has to retighten them if they are initially applied on tires that have heated up. Can't fight physics, there is no solution.
However, your fears are also correct.
You shouldn't rely on x-chocks alone. You can make your own or spend the 5 bucks on the cheap plastic ones, or get the higher grade versions that truckers use. Lots of options, from free to infinity.