If you add a large surge of people or +pages to your circles within a 24 hour period, the system will temporarily stop you from adding additional people even if you’re under the total number of people you can have in your circles. This is to prevent spam accounts adding huge amounts of people in short periods.
Duplicate profiles don't count towards the 5000 profiles or Pages in your circles limit!
If you have a profile in several circles that still counts as one in total.
More reasons for circling difficulties:
What about communities, do you post a lot to them? Are some or many posts removed by moderators (wrong topic for that community) or flagged by Google spambots?
That is one big reason why a Page gets put under the magnifying glass. Spamming other people's comments with links is another trigger for limited circling capabilities.
One way of getting rid of the limitations is to post good content in public, try to get more people to circle you. Engage.