They wouldnt necessarily get darker in a small amount of time, even same brand tests. Ive done 2 tests from the same pack dipped in same urine, one had alot stronger bfp than the other one, guess the amount of dye in each test can differ. Also these tests might not be that sensitive. I reccomend the asda/morrisons own brand (they are exactly the same) they are pink dye and work at 15hcg. Which are good if your unable to get an early response frer. 2 for £3.50