Checkboxes don’t work

I am creating a form for my business and whenever I submit a form the pdf has no checkboxes checked, no matter what I open it in the only difference is if I open it in some services then the checked boxes are just gone, they still don’t have any checkmarks they just disappear. Can anybody tell me if there is something simple Im doing wrong or has anybody else had this issue. I created the fillable pdf online with this website if that matters: https://www.sejda.com/pdf-forms The page I need help with: [ log in to see the link]