It's possible that the nurseries are correct. Flower Carpet roses stop blooming once in a while and may not flower for weeks. Normally, I'd suggest that instead of taking action, you wait to see what will happen to your plants. Because the leaves are turning yellow, however, you should be concerned. Your location may affect the health of your Flower Carpet roses. If you live in a region that experiences cold winters, avoid fertilizing late in the growing season. Late feedings promote the growth of tender, new tissue that can easily be damaged by cold winter temperatures. Instead, roses need to start hardening their growth in fall. Stop fertilizing your roses about 30-45 days before the date of the first frost.