You might not be hitting your ovulation time...have you tried doing OPK...maybe doing those would help you out...i wish you the best good luck :)
Most women ovulate about 14 days before their period is due. A fertility test will only show you as fertile around that time, some only within 24 hours before ovulation. They work by detecting an LH surge, which happens right before ovulation, from what I read within 24 hours hence the positive showing within that time frame. Most of those tests come with information telling you when to test. Some women have LH surges while they're asleep, or during the day when they haven't tested or already did. I never got positives using them, and I've been pregnant twice. Getting a negative on the test does not mean you can't concieve just that it doesn't detect a surge right then. It can take a healthy couple up to and over a year to get pregnant.
Umm...I think it can take up to a year to conceive before a doctor considers it a problem. I don't know anything about those tests but if you're concerned you should probably ask your doctor the next time you go in. Each month, even if both of you are fertile, you only have I think a 20% chance of conception. From what I've read there's only like a 2 day span every month where conception is even possible, and most women have no clue when that is (I sure didn't). Really, it's amazing that so many children are conceived out there, but if I were you I wouldn't worry yet, just keep trying.