Can you elaborate on that? Are you saying that you are unable to get your partner pregnant?
How long have you and your girlfriend been having regular unprotected sex?
How many times have you noticed only a small amount of ejaculate?
It's not unusual for a man to notice smaller amounts of ejaculate from time to time. It can be related to anything from when he ejaculated last to him not drinking enough water.
The amount of sperm is not directly related to the amount of semen. Meaning, if you ejaculate a lot or a little, it most likely contains approximately the same number of sperm cells. The issue would be if there is enough sperm in your semen, and if those sperm are healthy, normal sperm. Only a laboratory test would be able to determine that.
If you're concerned about your fertility, there are specialists that can help determine and possibly fix the problem. They can also evaluate your girlfriend. However, most specialists do not consider infertility until the person has been having regular unprotected sex with the same person for a period of 12 months without pregnancy occurring. Twice a week for only 4 months wouldn't qualify as infertility. It's quite possible that there is nothing wrong with either of you, just that pregnancy hasn't happened by chance yet.
I hope I've answered all your questions. If not, please ask and I'd be glad to clarify. You're not expected to pay for an answer until you're satisfied. I'll be glad to leave you a positive feedback once you've accepted.
Within reason, yes. If you ejaculate only a drop, then that's abnormal. But if sometimes there is a decreased amount of semen, this should not effect your chances of fathering a child. That possibility depends on the amount of actual sperm cells as well as the health of those sperm cells.