If you've detached it before this time, and used that method then maybe you have done something to your 510 pin connection, it may be bent from the fall or something. If this is your first time trying to detach the tank, period, then yes, make sure you aren't just turning the tank around, although the symptoms you are describing sort of sound like your 510 pin got stripped somehow.
How old is it? Also, if it sat for a while, with liquid leaking down in the battery part, I'm not entirely certain you may have somehow "gunked" up the 510 connection, but generally you want to get to that stuff out of there.
It still works though, that might lead me to think your technique is faulty as far as unscrewing it. It's the center pin of the tank that you want to get moving, so yes, I guess make sure you aren't just spinning the tank's glass or whatever.
Although honestly, this whole situation is somewhat confusing. How'd you knock it over, how bad was the fall, and if there is liquid leaking down, it's very possible that your tank glass is cracked or broken.
This is one of those times where I'm going to suggest returning to where you go it (if it was a vape shop) and ask them to help you with it. I'd say watch a video but it doesn't really suit the situation, so much, since I doubt you'll find something really helpful. Returning to the store with your purchase (and I would probably be up front about dropping it and etc., because they need to know to watch out for possible broken glass and etc. I guess. Etc.
Anna