This applies to a girl, but also seems like something that can be related to anyone that you have contact with regularly without attraction

Start of the semester: Met a girl in the year above me of my graduate program, and from the start seemed to be vibing. She smiled at me, was eager to give her number, and we had engaging conversations when our cohorts would get together for different parties or events ( i would do light kino such as pinching her hips, she would follow my lead when going from place to place). I did hit her up one time but got no response as it was sorta late and the group did hangout the night before.

Current interaction: We only make brief eye contact, she seems to ignore me but I can tell that she notices when i walk into a room or is around. When in a group conversation, she tends to comment back at what i say with sarcasm ( i assume a shit test). She did recently mention wanting to go to something that i do with my roommate every so often.

The specific question is what causes the flirty interactions to become awkward or go dead? The more general question is what causes a good first impression to change into more apathy for a specific individual?