Did you mean to say that you planted them the last week of May?

I'm not Zenman but you did not give enough info. Were these plants or seeds you started? Pinching them back only makes them bushier it does not make them grow. When you remove the top flower it sends out more shoots so it bushes out.

Where do you have them planted. Zinnias are a full sun plant and if you have them in the shade they won't grow as well. I never pinch back Zinnias. I enjoy the first flower and remove it when shows signs of deterioration. Then my plant bushes out and I have more flowers

I'm confused by ladyrose's post, as well.