Yamaha MT125 – does not wheel with warm engine

Hi, I recently hit Weelies with my MT-125. At the beginning of the ride the machine goes up well and I can easily bring it to the tipping point. However, if the engine is longer and I have already driven 20-30min, I try the Weelies again. The problem: the machine hardly comes any more no matter how high I bring the speed. It only goes out of the springs and only a few centimetres high. The acceleration however remains before and after the rides. I have e the feeling that the gas is less packed after a longer journey. Does anyone have similar experience/solution suggestions? Is this perhaps also normal? I would be happy about help. Thank you