Cosmos DB Change Feed Trigger: Stops polling for changes in between

We are pushing data to cosmos db collection from multiple threads(task, 2000 records in about 20 sec and each record if of 4KB in size), all the data is successfully inserted in to collection, But we have cosmos db change feed trigger which reads newly inserted documents process it and and inserts in to other cosmos db collections. so after processing the other collection should also have 2000 records but when i check it has only 998 records, after verification i came to know that in between azure function has missed few records.

for ex: in sequential incremented data if last processed number 1532 next was 1610 records in between these two numbers were not processed.

so to test i created sample cosmos trigger azure function just logged count of change feed it is was receiving using static variable which outside the run function and class. total change feed read was less than 500.

throughput of the collections were 5000 each
azure app is in app service plan slandered S2

What could be the issue?