What variety is it? The sterile Lantana cultivars definitely bloom for a good deal longer than the species types, which, while still "long"-blooming, will definitely slow down as they start to set seed in the heat of late summer.

I've found in my limited experience with lantana that it tends to bloom in cycles -- it will put out a flush of blooms then rest a bit before another flush with maybe one or two scattered blooms during that rest period.