I recently had to replace an inner tube, and what should have been a simple, quick procedure ended up taking me over an hour. The problem was that when I went to go fill the new tube with slime, the tube wouldn't take it. The value core was out, the valve stem didn't seem to be pinched, and the tube would take air. After fighting with it for a while, I decided to try another tube, and that tube worked fine.
Since it ended up costing me so much time, I decided to do a postmortem on the tube to find out what the problem was. When I cut open the tube, this is what I found:
[IMG]cyclingforums.com700Open ↗[/IMG]
It seems like the manufacturer offset the valve stem from where it was supposed to be placed. Here's a closer view:
[IMG]cyclingforums.com700Open ↗[/IMG]
The little flap there at the top seemed to be blocking the ingress of the slime. For comparison, here's a good tube:
[IMG]cyclingforums.com700Open ↗[/IMG]
The valve core is removed, and you can clearly see light through the valve stem.
I hope this post helps saves someone some time and frustration.