Problem Updating contacts

I already imported contacts from another database with their customs fields, but today I decided to add their id from the other database to do a weakly update where I will compare both databases and create a csv file with the updated data. To do this I exported all contacts, looked them up on my other database and added their id on the keap list and then I imported them on keap, Out of 27k contacts I exported from keap, on my list I have 27k contacts with this custom field that already existed as a custom field, but only 8k have the field after the import, why is the reason on that?