Legacy Landing pages use "layers" you position on the page.
The most common reason to have a form that you cannot fill out on them is that there is a layer above the form layer blocking anyone from filling out your form. Dive into the landing page editor. Once there, drag/re-order your form layer so it's above the layer blocking it.
Note that this is NOT a problem you will find with ONTRApages or ONTRAforms, and only exists in the older Legacy Landing pages!