I used this when we were TTC........and here's a thing I noticed: if I left them to work on the bathroom bench which is granite (and very cold) they took ages to show the lines, and if I left them on the bedside table the worked quicker and the lines were a lot clearer.............

So I think temps in the room may actually affect these?

I have heard that it's the same for HPT's, so whilst it might be my imagination, may be worth bearing in mind...........