Thank you for your post.
Without seeing your database/layout/fields, it is difficult to determine what is the problem.
Let's assume first it is a corrupt portal. Create a new portal, exactly like the first, and attach the five fields. Does this have the same behavior as before? If not, then you can remove the old portal.
What field type are the three fields that are not working? What is the key field(s) that make up this relationship for the portal?
Any additional information you can provide may be helpful to help determine the cause.
Yep. It was a 'corrupt portal'. I have no idea how that happened or how I can prevent it in the future. Is there a 'likely cause'?
I'm glad to hear everything is working.
I don't think there is any "likely cause". It could have been memory at the time of creation, incorrect writing to the file, another object on the layout that overwrote some of the portal information,.... For every possible cause, there could be hundreds more, so I don't want you to focus on just one or two things in case this happens again. The bottom line is that it shouldn't happen. Then again, I don't know if the portal was touching another part (Header, Sub-Summary, etc.) which may bring up some other strange behaviors.
Sorry I can't be more definitive, but I would not spend time worrying about it. I don't want you in the mindset that if a portal doesn't work properly again, it is "corrupt", because it may not be. Everything seems to work now, so my advice is to just move on, and when you encounter another problem that you cannot figure out, come to the forum and ask for help. :-)