Like I said, some are manufactured better than others. Your phone may have simply come with a crappy battery. It's always worth investigating every possibility.
Also if you only bought the phone four months ago, it should still be under warranty should you end up needing a replacement battery.
Try and find someone with the same phone as you. Try their battery. If it works, then you know yours is garbage. If it still doesn't work, you know it's not the battery but an issue with the phone itself.