I own that charger, and the slides aren't terrible, but they did ease up over time. I agree with the idea of sending it back if one slide is THAT bad, however, as if you are having trouble with it, you may well damage the battery.
I have found with this charger, the best method is in fact to slide the slide as far as it will go, insert the battery, and then gently "release" (but not completely, just reduce pressure) so that it slowly makes contact with the battery.
Best of luck with a replacement, I would personally go that route, probably.
Anna