LH line gets darker days after testing?

Emily

Hi all--this is my first time using ovulation tests. I've been testing for 7 straight days and a few minutes after I test, the test line is always fairly light compared to the control line. Some of the test lines, though, have gotten darker after a few days of sitting out. Is that a normal thing when tests sit for a few days? Or was that my LH surge and it took a really long time to show up?