What causes pump?

[Dont wanna be old]

How come sometimes i get a sick pump at the gym and sometimes i almost get nothing? i hate that feeling so i figured i will ask for i can change what ever im doing wrong

I don't know if you tryed any OTC supplements . Vandryl sulfate was the first supplement in the 80's that actually did something . Really didn't add muscle , but made you vascular and crazy pumps . I tried with creatine once that was introduced . Was like 1 + 1 = 3 . Now they have all sorta nos supplements .
Anyway , check out the VS for like 15 bucks for 2 months supply . You might feel like guy who's arms exploded .