Some of the themes that come with bootstrap studio don't have the footer pinned to the bottom of the page.
The instructions in this link are helpful here https://forum.bootstrapstudio.io/t/footer-always-at-the-bottom-of-the-page/7517
Basically, create custom sitewide CSS (ie. through a .css file under the styles folder of the design), with the following:
body {
display: flex;
flex-direction: column;
height: 100vh;
}
footer {
margin-top: auto;
}