Thyroid, your lovely cycle, timing.. timing.., ovulation., weight, activity, body fat, healthy "issues" etc. SO many things take play.. (& no its not a fun game)
Some women are lucky and can get pregnant right off the bat.. for most, its not the case..it can take several months, and or years.
one thing that worked for me.
get it. use it. love it.
& no.. you aren't guaranteed to have twins.. haha.. FYI: that lovely gene runs in my family...