Problems with backup fairphone agenda to google agenda

I have problems with backing up the agenda of the fairphone. I want to make a backup to the google agenda, but whatever I try three of the eight agenda’s in fairphone don’t get in the google agenda back-up. Today i was on the phone with a guy from fairphone support but also with the help of support no complete backup was realised. So the regular solutions to the problem don’t work. Has anyone an suggestion how to fix this? I like to thank you in advance for your reactions!