Unable to place Form field in document.

[defaultxzllrrha704v]

Have form that I have used form long time now cannot put new form field in place. Also some other fields are now not editable.

[Thom Parker]

If the PDF Form has been edited many times, then it's likely to have been corrupted to the point were Acrobat can no longer reliably modify the AcroForm entries.  

You could try saving as Optimized, and performing a repair. But it may also just be that you should recreate the form from scratch.