Mongodb sharding - set shard size
Mongo can't determine key ranges for sharding if you use increments.
Here's an example:

You have a collection with keys 10, 20, 30, 40, 50, 60.

If mongo have to create two shards, it may assume the keys range as: [10,30] and [31,60] (or similar).

By continuing to write bigger keys, they will always go to the second range.

Mongo will adjust the ranges, but it can never know what the next key will be, and it will always put it into the last range.

Try to use some key with good distribution.

More details here: https://docs.mongodb.com/manual/core/sharding-shard-key/#monotonically-changing-shard-keys