Solid smiley face result in one test then another test no LH surge

Question: I had a peak fertility solid smiley face today (day 15) after 3 days of the high fertility flashing smiley face, after a few days of the low fertility blank circles and noticed that  when I pulled the stick out of the test holder that the result line was fainter than the test line which the instructions say don't read the stick lines which is fine but when I did another clear blue analog stick test for back up the same time today (same cup of urine) the result line was also fainter then the test line on that test so I don't know why I would get a peak fertility solid smiley face on the clear blue advanced digital but not get a LH surge on another test from the same brand, has anyone else encountered this problem or is it just me?