I rarely cum from getting a blowjob. Good head gets me horny as Hell and I want to fuck like a mad man but the only way I know I will cum is in a woman's pussy. For me NOTHING feels better than a wet pussy wrapped around my cock.
You may never have encountered a cocksucker that truly loves sucking cock. They are out there, so don't give up hope.
However, if you are hung really thick, even the most dedicated cocksucker is going to have difficulty satisfying you as they need to have some room in their mouth to work with. If your dick is really stretching their mouth open, they have no space to swirl their tongues around in, it's hard to create suction and almost impossible to swallow your cock down into their throat. Basically, all they can do is bob up and down on the head and maybe an inch of shaft and give it a lick or two when they come up off it or just lick it like a lollipop or ice cream cone.
Not very likely to give a great orgasm, so if you're really thick, you may have to settle for getting good pussy and just enjoy their attempts to give you head as an appetizer.
I consider a good blowjob as a part of foreplay not an end unto itself. I always have thought that bulls that are big on cumming from oral sex are most likely a bit selfish and/or too lazy to want to satisfy a woman. Unless of course you get hard right away and can fuck longer after the oral orgasm.