LH doesn't metabolize into your urine overnight, and you are therefore more likely to catch your surge by testing later in the day. If you were getting FMU positives, I'd hazard a guess that that was when you were on Clomid, as Clomid increases your LH levels. Higher level = more likely to be a stronger line, even if the surge is later in day.
I test twice a day....once mid morning and once about tea-time. About 75% of the time its the teatime test that picks it my surge.
Have you noticed your lines getting darker, if not positive?