The creeping thyme under the white bowl has not spread one bit this year. It's green and healthy, but not filling in any bare spots where I had removed perennials last fall. 
|  | 
| Perennials got overrun by the creeping thyme | 
The thyme overwhelmed each, growing right over each little emerging perennial every spring. The creeping thyme completely choked everything out.
So I removed the plants, letting the thyme spread where it will.
But.
No action at all this year. I replaced the birdbath with the white bowl. I watered in the bare spots all summer, added compost, encouraged it. It hasn't spread anywhere all summer, not even an inch.
|  | 
| All summer it has looked healthy but has not spread | 
Why did it spread so quickly and aggressively before but has stopped at its existing boundaries this year? I'm puzzled.