I was going through my contacts on the Google contacts app. For some reason I have some contacts I can add to my Google+ circles and others I cannot. Any idea why not?
After read your question, I just want suggest to you read a manual help support from Google apps/contact or G+. About add people to your circle from contact.
I just found this https://support.google.com/plus/answer/1047805?hl=en&rd=1&authuser=0.
Google says, any some limit for adding people to your circle on a day. So you must wait for a next day to do again. I copy this for you:
Limits on circles
- You can add up to 5,000 profiles and pages across all of your circles. There is a daily limit for how many people and pages you can add to your circles. If you reach the limit, just wait a day to add more people or pages.
This my own contact, and I find too some of friends I can't add to my G+ circle.
After I look for detail, my friend not using Google product (or) account. So absolutely I cant add them to my circle.
Hope you can get more answer after reading rule or mechanism about how the Google apps work from link I choose it for you.