Ovulation Tests

Is it normal that I would have a positive ovulation test only in the afternoon and then not the next morning? I use the Clearblue Digital tests and in the past I have always gotten my positives in the morning, but last month I didn't get a positive (I am on clomid), so I started testing twice each day. I usually stop testing after the smiley face, but I was curious if it would still be positive the next morning...and it wasn't. Does anyone know what kind of timeframe your LH stays increased to detect the surge? Any advice is appreciated :)