That's not an easy question to answer! But a very good one! I think it ultimately depends on why it wouldn't pull. I have never been successfully in tossing it back in to the smoker (or oven) after pulling it off the smoker and letting it sit or rest (the only time I haven't pulled immediately has been because I had to leave the house for one reason or another) - it either gets over-cooked (the meat mushes instead of pulling) or tastes like it came out of (ewww) a crock-pot.
When I do have to pull later and the meat has tightened up I find a finishing sauce (a vinager-based sauce) lightly tossed in to meat after pulling or more like tugging (after more effort than I'd like) helps bring it back to the level of taste my hot pulled pork. And is beneficial in helping maintain a nice moisture content when stored for reheating later.
Keep at it! For me it's become an addiction. Two years ago you couldn't even get me to touch pulled pork, now I smoke, at the very least, a butt every weekend. Now, I still won't eat BBQ, unless of course I'm the one that made it and neither will my husband (I bought HIM the smoker for Christmas last year, but he's not allowed to touch it! He's the BBQ fan), my coworkers, neighbors or friends!!