2wheels,
OK, given that you grease it every 1000 miles and that it is a bit cool out, it could be that the grease already in there has stiffened up a bit.
The grease you are using is a possible concern. If you are using different types of grease every time you add some, maybe the new grease is reacting with a different type of old grease. I know that is far fetched, but a slight possibility.
I would go with the suggestion to clean the zerk fitting and make sure it will pass the grease.
Also, you might want to take the bike for a ride and after the suspension has worked a bit, then see if the grease will flow in. Based on the theory that if the swingarm has been worked it will warm up a bit and cause the grease to get warm and maybe loosen up a bit.
Just my thoughts, and what I would do if I had the problem.
Charlie