If the load does not specifically call for buffer, then I would NOT use buffer; to do so could raise pressures to a dangerous level.
What are the particulars of your load (hull, powder charge, wad, ect). If you are using the regular plastic wads for reloading shot, you may have to find a differnt load. I used some older peters "blue magic" hulls a while back, and they just wouldn't crimp right....left a big gap in the opening. If that is the case, you can try and remedy the situation by using a tyvek patch, available from BP or Precision reloading. Other than that, you might just have to try a different load combination. One trick you can try (if using standard plastic wads, such as the remington or Winchester type), is to compress the "arms" of the wads so they have a little more "give" when crimping. Load up the hull as normal, but instead of putting in the buckshot, fill the wad about half way with birdshot and seat with the rammer tube of the reloader. Then dump the shot, add the buckshot, and crimp. Hope this helps. Good luck.