Hi,
You have rendered the pdf as pdf/a. Acrobat/Reader will automatically view the pdf in pdf/a mode, which means non-interactive mode. You will have to open Reader, go to Preferences and under Documents select never on pdf/a view mode.
Better way is to save the pdf as standard pdf, not pdf/a. This way each client doesn't have to change the preferences to be able to fill in the form.
Regards
Daniel