"Stretch" could be either literal or figurative. To "stretch" something is to pull on it so that it becomes longer. You "stretch" a rubber band. You "stretch" your money and "make it last longer" by spending carefully. Muscles and tendons are stretchy. In fitness and athletic training, you do a "warmup" before intense exercise, and part of the warmup consists of stretching them carefully. "My coach does not stretch me enough" could mean "my coach doesn't take enough time on stretching during warmups," but I think it means something different. "A stretch" can mean "a goal that can be achieved only by unusual effort." A trainer or coach could, figuratively, "stretch" somebody by setting goals that are difficult, but not quite impossible to achieve. "What's your best time running a mile?" "7:15." "Today, is the day I want to see you break 7 minutes. That's going to be a stretch, but you can do it." I think "My coach does not stretch me enough" means "my coach sets goals that are too easy. I don't progress as fast as I could."

Hello Wendy, Without more context it there are two possibilities. The most likely is that your teacher sets you work which is too easy and does not require a lot of effort on your part. Best wishes Bob

This is a really awkwardly constructed sentence. The word 'stretching' is referring to stretching exercises to 'warm up' the muscles/provide flexibility before an activity/work out/game/match. It would be clearer by saying, 'my coach doesn't do enough stretching exercises with me'