First, if you are not very inflexible be careful about stretching before exercising. You said you stretch during warm up, so I am going to assume you are stretching once you have already done some exercise and are actually warm, which is good. If you are stretching before you even start, you could injure yourself.
It sounds like you are doing all the right things as long as you are always stretching warm. Once you have worked out, stretching at the end is the most beneficial. You reduce the probability of have sore muscles, and you should be increasing flexibility. Remember to hold the position, don’t bounce or go quickly back and forth when you stretch. Some people feel they get a better stretch when they do floor or bar/barre stretching rather than typical stretching done standing up.
I am very inflexible, I know how frustrating it is. I wasn’t as a child either. I do gain flexibility when I work on it. I am more flexible now that I exercise regularly again compared to when I was doing nothing, but I still am fairly inflexible compared to most people, especially compared to women.
You could try yoga. I personally dislike yoga, but a lot of people love it. Ballet and Barre class will increase flexibility also if you want a whole class that works on strength and flexibility.