Weird scrolling with pop up plugin on mobile

Hello!
I am having some weird issues with the pop up plugin. I love the plugin and it is doing exactly what I need it to do, it just seems that when I access the pop up on my phone, that sometimes there is a clash when I am scrolling as the backdrop scrolls and not my pop up.

Go to voltaoil.setteradvertising.com on a mobile phone... I am using the iPhone 6. Select the Order Fuel option from the dropdown mobile menu. When the form "pops" up try to scroll. It is not smooth and sometimes the pop up doesn't scroll, but the page behind the pop up does.

Can you please help me fix this so the pop up scrolls smoothly? Thank you for your help!