My primary use of Acrobat is to fill out forms. If I fill out a form and come back to it, sometimes I can't change what I entered before. I hit edit and the blue out line box comes up. When I place the curser next to what I want to change, I can't change it. I can add but it will superimpose over the what I typed before. The item I want to change can't be manipulated in any way.
We apologize for the delay in response to your query.
Are you working on Adobe Acrobat or Adobe Reader?
The form on which you are working on, could be an XFA form created in Adobe Livecycle Designer or using a different pdf producer.
You can check the property of the document, right click on the file and "document property"