I checked pages both of you and there is one thing in common. You are using custom registration pages. It will not work automatically in that case. It would require you to manually add Math Captcha code into the registration forms and make some changes in Math Captcha plugin.
We have in our plans to make it easier (or possible at all) to use MC in custom forms, but it’s hard to say when it will be ready.
How to Get Support
After you register and login to the site, you may post all plugin support questions in the Support Forum.
If you need to provide private info, please create a ticket and then reply to it using Set as private reply option.