Disappearing lines

I need help. I took a pregnancy test (first response) on Thursday 8/11 and it was an obvious positive. I took another one on Friday using CVS early pregnancy test and in the first minute it came up positive. I went to look at it a couple hours later and it faded a lot. So I took another one (same brand) and it did the same thing. It came up positive within its suggested time frame but it disappeared about 12 hours later. Any idea what's going on? Could it just be the brand? Oh, and the first one I took (First response) did not fade away.