Cannot add Radios or Select list fields

I'm trying to create a new webform that uses some radios and select list fields. Whenever I try to drag the radio field or select list field over to add it to my form, it will not show up. It will say "Please wait...", but the field will not show up in Form preview. All of the other fields work (e.g., e-mail, textfield, textarea, file) except the radio and select list. Any ideas on what may be causing the issue?