what am i doing wrong?
You arent doing anything wrong. There are some atties that have shorter 510's than others and without an adjustable positive pin, that atty wont work with that reo. Make sure you dont have anything between the atty and the mod (like an o ring). You can try to twist the base on as tight as possible (tighten, loosen and tighten again) by hand and then put the cap on. I have had caps prevent the atty from screwing all the way down. Switching batteries wont help this problem tho. If you trust yourself with a grinder, you can take the catch cup down a bit but it might be a better idea to either send it in for the new adjustable positive pin or get a different atty.