If the pin has been neglected for some time, dirt and grease could have hardened, preventing grease from entering. If the zerk fitting is OK you may need to force grease into the zerk. Here's a link (http://www.northerntool.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=6970&productId=200130974&R=200130974) to a tool which Northern Tool sells.
And, of course when all else fails, you take the pin out , buff it down clean, pump freash grease through the pin, and reinstall it. I have had limited sucess with the grease blasters. Unless you put something in it that desolves hardened grease, or heat the pin up before using it.