Lagging Zucchini

While this photo was taken about a week ago, my zucchini and summer squash plants have not really grown much bigger than this. As you can see, this is that last 20% of the veg garden that has not been weeded yet. Could the crab grass, clover and other weeds be the cause of my squash's slow growth?

The zucchini are normally filling the kitchen table by this time of year. I bought plants this year, got them in the ground at a reasonable time, but they aren't doing very well. The soil here is decent, and I've never know zucchini to be that particular. My mom is having the same problem and we did get our plants at the same nursery. But one of my coworkers was talking about here zucchini harvest already.

Is anybody else out there having trouble with their summer squash?