First question, is the identification correct? Second question have you pruned it and if so how much?

Also, where are you located geographically, and what are the current growing conditions (soil type, moisture, light, competition from other plants.)

How long has it been in place in your garden?

Adding photos of the plant, with one showing a close photo of the leaf, will help check the ID and also help evaluate what might be the problem. This is typically a vigorous, free-blooming plant once it is established.