Clear Blue Digital OPK ?

A

I took x2 cheap OPKs yesterday & were negative, had egg white CM.

Today once I got home I bought the clear blue digital OPK & it came up with a smiley face. First time using this brand. I know the smiley is s LH surge. No egg white CM today.

I did another test a few hours later just because & it came up as negative.

Anyone know why ?