Sorry to be the bearer of bad news! They do work differently to pregnancy tests. There are always two lines, with one fainter than the control line. An LH surge is indicated by both lines the same colour as the control line. However, doesn't mean that BFPs aren't on their way - good luck!!!
I don't always get a faint second line, yesterday and today I only had the control line (day 9 of cycle). Last month (first month of using) I only got the faint line just before and just after ovulation. Am I doing it wrong?!
It depends per woman, some always have a bit of LH in their system which will show up, others have lower levels. You can't always tell if your surge is coming up either. Only a second line as the control line can tell you for sure you're having an LH surge.
I get a faint line most of the time. I'm 5 weeks pregnant and did one yesterday to test the opk as a hpt thing and it's definitely true. Line went dark as soon as it got wet and I've never had one go so dark so fast. If you are pregnant the opk should get darker over the next few days I think.