If you are using HTML tags inside form design, especially iframe, always close <iframe> tag with </iframe>. Do not close it as empty tag using <iframe/>.
If you are using <iframe/>, you might find that the right click context menu to insert/delete/modify columns & rows does not work. Similarly, the one-to-many reference related functionality also may not also work. To fix this, look for empty tags in your form and modify it to have separate closure. e.g: <iframe></iframe> instead of <iframe/>