I've only had trouble fitting with two women, and they were both short, petite women.
The first one was only 4'11" tall. It was our first date, and after a nice dinner out, she invited me back to her place to watch a movie. She initiated the sex, but maybe wasn't ready for it, because I just couldn't get inside her. She put lube on me and we still couldn't get me in. Then she tried to give me a blow job and her mouth was too small, so I just got a hand job. We went out two more times but never had sex again.
The second one is my current girlfriend, who is 5'0". I couldn't fit in her the first time we tried to have sex, but again, I think that she was too tense. Sometimes we don't have any problem at all, and sometimes I can't fit inside of her even after putting on some lube. I always give her plenty of foreplay, so I don't know why it's sometimes so hard.
Good decisions come from experience. Experience comes from bad decisions.