1. When the form (DataPage) elements are not being referenced correctly.
To refer to the correct field of the right DataPage use document.getElementById(“InsertRecordFIELDNAME”) rather than document.forms.InsertRecordFIELDNAME.
2. When the form submission is not referenced properly.
To refer to the right DataPage, use document.forms[X]. X starts at 0 which refers to the first form on the page and increments based on the order. For example to call onsubmit event of the second DataPage on your page use the following:
Another useful resource is http://www.w3schools.com/js/default.asp