@greg67 If you're aiming for muscle ups, not sure where you are aiming on your way up, but you should try and go for ones where you get the bar to almost your sternum, or as close as possible.
You want to keep your legs straight and slightly out in front of you and minimize any swinging or kipping(until you're ready for muscle ups, then kipping becomes necessary, but perfect your dips and pullups first so you don't hurt yourself.)
I think this checklist from Athlean-X is fairly decent to make sure you're doing the best form possible:
Finally, go until real exhaustion, it's bodyweight, so you get a lot more leeway than if you were using weights. Go for something like 30-50 reps total before you quit(okay, maybe a bit hardcore, but maybe 3x your max reps and then go for bigger goals, you want to feel like you can't even do 1 after a minute rest). So if you only get 7 reps in the first time, rest a minute or two and go again, rinse and repeat. Unless you feel actual sharp pain, just keep going. You need to tear down that muscle and rebuild it stronger. If you're only doing one set to 3-8, it makes sense why you're not progressing.