I don't know what your specific situation is. I guess it's because your sports coordination is not enough. For example, bench press, in addition to the strength of chest muscles, triceps and deltoid muscles are also very important. Now I can't get on the bench press until 200. The reason is that deltoid muscle is weak, so I practice deltoid muscle more now. Also, I don't know how your endurance exercise is. If you always repeat with heavy weight, it will only increase your absolute strength and have little effect on endurance exercise, so what you said will happen.