Chunks not splitting on MongoDB sharded clusters
Your chosen shard key for a collection defines the granularity of possible chunk splits. Chunks represent a range of values for the shard key. With a single field in your shard key (TxnMonth), that means the smallest possible range is a single value such as TxnMonth: 3.

The chunk size setting determines when MongoDB will try to split chunk ranges in order to help distribute data in a sharded cluster, but does not put an upper bound on the size of the data in a chunk range.

When the maximum chunk size is reached for a given value, that chunk will be marked as an indivisible  jumbo chunk and the balancer will no longer attempt to split or migrate that chunk. New data will continue to be added to that chunk leading to an imbalance in data distribution as you observed in your testing.


  there are 8 distinct values of TxnMonth from 1 to 8


This limits your sharded collection to a maximum of 8 chunks (or the number of unique TxnMonth values in this collection).

For effective sharding you need to choose a more appropriate shard key with higher cardinality.