why are forms not signable in reader app

I created a PDF document with no security setting enables.  I allow for all features to be used.  The documents worked just fine for years.  Now, when opened in the phone app there is a lock in the bottom right corner that will not allow for signing.  All the other features still work including, markup, highlighter and Type.  I have save these documents with reader extended pdf with enabled additional features.  Does anyone know why this is happening?