Ok so in short i am 16 years old. I work in a grocer. Now i have been practicing all sorts of technics and i've been pretty solid on them. I can use them on just about any girl without many problems. However there is this girl i have recently been talking to. I didn't really think anything of it and i assumed i would just do my normal thing. Unfortunately that is NOT what happened at ALL. She came through my line and for whatever reason my heart dropped into my stomach and i started getting all weird. This has never happened before, it's like my brain completely turned off i started losing eye contact i was monotone, in short it was unbelievably bad. So my question is, even though I was SURE i could handle this, WHY is she different from ALL the other girls, why did this happen with her? It's very unusual because i can usually carry myself very well around chicks. So any idea of what happened?

TL;DR I am decent at escalation but for whatever reason I couldn't focus enough with this particular girl.