Does the morning-after pill work if the woman has already ovulated?

I’ve been doing some reading about this. Apparently it works by preventing ovulation. So what happens if the girl has sex when she’s ovulating, or a day or two after? I assume the egg is already there so will it not work?

This pill’s success rate seems low when compared to the regular contraceptive pill and I’m wondering if this is the reason why.

That is correct. According to the Planned Parenthood website,

The answer is, it depends on which version of the morning after pill.

The application for approval for the morning after drug Ella in Europe stated several methods of action:

• ability to block, disrupt or delay ovulation
• ability to block or delay ovulation even after the onset of the LH surge
• ability to delay maturation of the endometrium likely resulting in prevention of implantation

(See the bottom of page 9 of 49 in linked application)
Further, in animal studies it showed similar effectiveness as RU-486 at terminating established pregnancies.

So it seems that the drug’s manufacturer thinks it may work even after ovulation.