Recent research has revealed the pterosaurs may in fact have been capable of powered flight.
First, the issue of pterosaur metabolism. Some recently discovered pterosaur fossils indicate that pterosaurs had a dense, hair-like covering, somewhat similar to fur. Hair or fur covering is typical of warm-blooded animals because those animals need to maintain a high body temperature when external conditions are cold. So if the metabolism of pterosaurs was more like that of warm-blooded animals and so faster than the reading suggests, then it would have supplied them with the energy needed for powered flight.
Second, the idea that large pterosaurs couldn't use powered flight because they were too heavy. We now know that pterosaurs had anatomical features that made them unusually light for their size. For example, the bones of pterosaurs were hollow instead of solid. Hollow, light-weight bones would have kept the pterosaurs weight low despite their large body frames. The pterosaurs' weight was probably low enough to allow them to keep themselves airborne by flapping their wings.
Third, takeoff would indeed be a problem for pterosaurs if they took off the way birds do. But there are important differences between birds and pterosaurs. Birds only use their hind limbs, their legs, for walking on the ground. So they only have two limbs to push off from when they launch. But pterosaurs walked on all four limbs while on the ground. There are modern flying animals that walk on all four limbs. Bats, for example. And they use all four limbs to push off the ground, not just the back ones. Studies indicate that even the largest pterosaurs would have had no trouble using all four limbs to run fast enough or jump high enough to launch themselves into the air.