Two thoughts

Xander stopped and thought for a moment, a near Herculean task when you consider just what Faith was doing with the hot cop.

He had seemed to get more and more exhausted as he used his powers. Now because of his comic knowledge two options came to mind. The first was that he was physically linked with his abilities. That meant that he was using physical energy every time he used his powers, too much too soon and he would collapse from exhaustion like he just did. Going slow and getting some rest he could still do what he had planned to do.

Option two was that he only had a limited amount of mind control power to begin with and he had just used it all. In that option he was screwed, but not in the way he was planning on.

The only question was how to test to see what possibilities was right. The way he had programmed Faith and the cop both would do anything he asked.

So he had to find someone else. Then again he could always forget his troubles for a while by fucking the two women who were in his power.