Losing erection

Victoria
My BF and I have been together for over a year now. We have a great sex life...but I'd say once every 3 times we have sex he loses his hard on. I will give him head to try to help get it back up and most of the time that works. Sometimes even after he gets hard again, it's like because he already got soft once, he keeps getting soft. 
I don't know what to do about this. It makes me feel unattractive and like I'm not doing something right. He tells me it's not me but I don't know what to do or believe.