Rarely, but once in a while I get an attack O2 won't touch. More often, but still rare, I get what you describe... O2 knocks it down but won't kill it. When I feel I am getting no further with the O2 I will get up and walk around for a few minutes, then when I go back to the O2 it works completely. As far as them going away while I am taking the break from O2? Never had that happen. I figure it could just be the hit ending normally for you... mine usually last longer than most. But again, everybody is different. There are no set rules with CH.

Keep in mind I have very limited experience with o2 but I have had similar results at times.  I normally have attacks that last hours if untreated.  With o2, there is a noticeable pain reduction over about 5 minutes, most occurring in the last minute or two.  After that it seems to reach a plateau around level 2 or 3 that doesn't improve.  I come off the o2 and in about 10 minutes the pain is completely gone.  I've gone 12 minutes on o2 with the same results.  If the pain isn't gone by 5 minutes I'm better off stopping and within 10 minutes it's all good.  I've tested this at 15, 20, and 25 lpm.  I may order the o2ptimask and see if that makes a difference since I now have two tanks (one for travel) and could use a second one.

This is actually almost exactly my experience-- o2 works almost as a preventative for me. I'll see one coming on at night, and then get on o2, and it almost vanquishes it completely, I come off, and then maybe 5 minutes later it's gone.

Out of curiosity, what are your triggers?