Teeth, too much suction, not enough suction, lack of creativity makes it boring, no consistency makes it difficult to achieve orgasm…
A lot of things can make a blowjob bad. As for how long it takes to ejaculate, it depends on a lot of things other than how good the person performing the act might be.
I’ve been with my husband for 7 years, so I’m pretty much in control of his orgasm during a blowjob at this point just because I know what he likes and I can read his body language and sounds he makes. If I want it to last, I’ll make that happen. If I want it to be quick, I know what to do to make sure it is.
Practice makes perfect.