The line will be fainter after ovulation because the tests detect LH (Lutenizing Hormone) which surges BEFORE ovulation. The tests are designed to show you when you are having this surge so you know when to start BD'ing as the window of opportunity after seeing this surge and ovulation itself is fairly short (No more than 36hours).
Obviously after ovulation has happened, you won't detect your LH in such high volumes, so it will show as a negative.
-But just because you now have a negative ovulation result, it doesn't mean you have lost your chance, if you are still within that window of opportunity, you can still concieve.
For example if you did an OV test yesterday and it was ++ thats when your surge was, but today you would still be fertile, but a OV test may be fainter today as you approach ovulation itself.
Hope that makes sense!