what causes low semen volume?

The presence of high levels of fructose (a sugar) is normal in the semen and this comes almost entirely from the seminal vesicles. Obstruction of this accessory gland will result in low semen volumes since the seminal vesicles normally produce 70% of the seminal plasma. A test that showed low fructose levels combined with a low semen volume would therefore suggest an obstruction in this part of the body.

There could be other causes like infection or injury.