Plugin does not work with custom theme

I installed and activated the plugin with succes.

When enabling the default contact-form within a new page, it works OK when using the default theme Twenty Fourteen

But our company website is using a custom theme, which I have to alter somehow. When running the same contact-form in the same page, I only see the text
[contact-form-7 id=”1292″ title=”Contactformulier 1″]

So there is no automatic translation from these tags/elements to the the actual contact form fields.
It has to do something that an automatic (search) plugin filter is not available or so in out custom theme.

Can somebody help me in activating the correct tag translation or that the “wp-contact-form-7” php will be executed ?

https://ww.wp.xz.cn/plugins/contact-form-7/