It might not be the same amount every time and that's normal. When it is less, have you recently had alcohol? That is known to decrease volume. This article has other ideas for you. https://www.medicalnewstoday.com/articles/326355#treatment You are a young man. So, IF it is anything psychological or any medicine you take, good to know about.
I'd also chime in, the volume might be less if you had masturbated earlier the same day, and also (I think I've read) if you are dehydrated.