I see what you’re saying.
Naturally you’re muscles will get fatigued if you keep using the same ones.
No disrespect, but judging by your post and the fact that you’re in the beginners section, i’d say your very inexperienced and would not benefit optimally from body part splits.
Also, if your workout is bench press, incline, then decline you’d be using less oxygen than going from bench press, squat, and then to incline or decline.(squats are more tiring than chest exercises so you will get more tired from them and just be all around fatigued as opposed to sore in a smaller muscle group)
The exercise with the most body parts used goes first is a good starting rule of thumb. (Ie squats go before bench in a full body workout)
You should check out some whole body workouts by Chad Waterbury, they’ll guide you in exercise selection and order as well as periodization.
It is normal for the lifts you do second, and third, and so on, to be a little weaker. As you gain more experience - and develop a solid mind-muscle connection - you find that this is less true. When I am incline bench pressing, I can hit my upper chest very well, and it has very little impact on my strength in a flat dumbbell bench for example.
If you have bodybuilding goals, I greatly recommend AVOIDING Waterbury routines and theory, and not doing “fullbody” workouts.