Hello @rkassebaum,
Thanks for asking. 
I see something quite weird in your website. To center align the button, it seems that you are using 3 column layout. First and third column being empty. On desktop you are not seeing the issue as button is perfectly aligning, but on mobile those empty columns are adding the space. Upon deleting the empty section it fixes the problem. Here is screencast. https://cl.ly/2N3Q2N0e0Y2B
What I would suggest you is instead of using 3 column layout, use one column layout. After that to center align the button, you can do that from Section > Formatting > Click Center Icon. And that will automatically fix the problem you are facing on mobile as well as it will be lot easier to maintain in future. I have recorded a screencast that you can take a look. https://cl.ly/1H153S2A1u0F
Thanks.