Fields are not saved when user joins another list

I have two lists setup for two types of users. Often users first sign up for list A, than a few weeks or months later they move on to list B. At list B I have several other fields that they need to fill in. Unfortunately when a user who is on list A signs up for list B, these fields are not saved to the database. (If user is not on list A, signing up for list B works perfectly.)

Is there a fix for this?