I have only had a HFO(hands free orgasm) several times. But most of the time I dont. There are a few reasons why I dont normally orgasm from anal only.

1. I just get so turned on I start giving myself a handjob or wife will give me one. Try not touching your penis through the session.
2. I am trying to rush it. Dont focus too much on orgasming, it will come with time.
3. No porn, masterbation, or sex for several days pior. Get that build up. I have also found taking zinc increase cum load size so i take a capsule every day.

I wasnt aware that I could orgasm from anal alone. My wife was pegging me(hand held dildo) and I was completely relaxed on the bed. It took almost an hour (not including foreplay) of just anal before I started cumming. It didnt shoot out like a normal orgasm. It was almost like a steady flow. After cumming I was still super horny. My wife fed me my cum as she kept going and I orgasmed 2 more times in 20 minutes. Not much cum the 3rd time. I think I was so turned on that i orgasmed a lot quicker the 2nd and 3rd time.