Confused about OPKs

Hi ladies. Anyone else using OPKs? This is my first month using them so I'm not too experienced. My fertile week started on the 13th according to Glow and I thought for sure I'd have a "smiley" in the digital display today since it's the 15th and Glow said 17tg is my peak.... BUT still not! Any idea what that means? Do you ALWAYS surge? I'm confused...thx