Sorry, also left out seminal vesicles which produce more of the fluid than the prostate. A problem there like an infection could reduce amounts. But look at medications. There are a few than affect semen production as a side effect.
Most of the seminal fluid is produced in the prostate, so a problem there could decrease the amount. An infection, or BPH (enlargement due to aging), blocked ducts due to inactivity, or
frequent ejaculation not allowing for recovery time. Also, lack of excitement and enthusiasm can result in a less than spectacular release.
A decrease in testoterone or a zinc deficiency would result in a reduction is semen volume. A deficiency in vitamins A, B or selenium can cause a reduction. Also, being very underweight or overweight can cause a reduction. Jacuzzi's, tighty-whitey underwear, laptops on your lap, bicycles, and frequent ejaculation are also potential causes.