Most times when I cannot add form layout it's because the layout's application and update set application are not the same - regardless whether you have the your update set current; so you may have to change you current application and create the update set with the same application, then change the layout for it to be captured in your current update set.

If you're using the popular community utility 'add update set' and you can find where layout is stored, you don't have play games with the current app - the utility will create a batch to handle different apps in an update set.