Indexes not used from some update queries

Working with Data
#1

I have a collection where there are lots of slow update queries. I have done indexing on the collection.
If the update query has {A, B, C} as the filter query, then the collection has the indexing (A,B,C).

Even though indexes are present, for some queries indexes are not used. I need to understand the root cause behind this?

#2

We cannot really help with so little details.

Please provide sample documents, sample queries, the output of collection.getIndexes() for all collections involve and the explain plan of the slow queries.

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