What could this mean ?!

So on the 25th I took my first ovulation test the line was there but not positive then on the 26Th I took another one and this one was darker and then the 27th I took another one and it was darker than the 26Th so yesterday the 28th I took another one and it was back to light like on the 25th so I though maybe it was a faulty test so I took another one an hour later and again it was light. According to glow I ovulated yesterday And I had sex on the 25th and yesterday. Why would the line be light now ?