sex position that didn't work out

So last night be and my boyfriend were having sex but I wanted to try something different. I really wanted him to fuck me from behind (not anal just like having sex spooning) but his dick wouldn't go in my vagina. His dick always goes in me in missionary but when I had my back facing him it was really hard to get his dick to go in my vagina. It's extra hard because I can't really open my legs whilst laying on my side. Do any of you have this problem??? How do you get his dick to go in you when you're spooning??????