Indexes not used from some update queries

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?

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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