fillable pdf not saving properly

Fillable forms created in InDesign can only be exported for use as Interactive pdfs. These pdfs can only be read by Acrobat 9 or above and even then I've found that they can be unreliable. You are probably better off creating your fillable forms directly in a print pdf that you've created from InDesign. They are far more reliable than ones created with InDesign as Interactives but you should be warned that there are some pdf readers out there that won't even recognize the forms created this way. A disclaimer should always be added to the end users that they should download the latest Acrobat Reader if they have trouble with the forms.

Just one added tip. If you have to make edits to your original InDesign file you can avoid having to do over the forms in Acrobat by first creating a new pdf from the corrected inDesign with a new name. Open the original pdf with the forms and click on the Organize Pages tool. You'll then see all of the pages in the pdf. Click on a page that you want to replace with the newly edited page and choose Replace from the options at the top. Navigate to the other pdf you made and choose it and in the dialog box you'll get in the replace with area make sure that you choose the page number of the page you want to do the replacing. The page will be replaced and the forms will not be affected.