I sometimes lose erection mid punt, but I reckon at my age keeping a hard on for an hour takes some doing. I'm OK with a half hour session.
It's usually down to one of several reasons,
Having a wank/sex too soon before a punt, say within 4-5 days.
The WG not being very attractive, attentive or just shit attitude.
Needing a piss.
It used to concern me, but now I just say, the old fella has got a bit bored, he needs a bit of stimulation, then either get more oral, or get her to give it a bit of attention, or I'll have to do it myself and tell her it's facial time!
If I need a piss, which I quite often do in an hour meet, especially in the morning if I'd had a few cups of tea, I just say, be right back and tell them to play with themselves while I'm gone.