Concurrent request on Elastic Search results null

Question

I'm trying to update documents in the elastic search using C# Nest. I have Azure service bus trigger function which supplies the data to be updated. Below are the steps I'm performing to update document.

1. Get the record from Elastic Search index by ID
2. Make the changes
3. update the record by ID

If I'm testing with a single message, the logic is working fine. Incase of concurrent requests, the Elastic search is retuning null response.

Below are the details of the servicebus message and Nest logic.

Message :

ID Data to be updated 12345678 XXXXXXXXXXXXXXXX

Business Logic :

public async Task Update(string message)
        {
            var obj = JsonConvert.DeserializeObject<Model>(topicMessage.Record);

            SearchResponse searchResponse = null;

            var esResult = await _elasticClient.GetAsync<SearchResponse>(obj.Id);
            searchResponse = esResult?.Source;

            if (searchResponse != null)
            {
                // Update logic which is working fine
            }
            else
            {
                throw new Exception($"Record with id:{obj.Id} not found in ES");
            }
        }

If I'm processing 10000 records at a time, then about 500 to 1000 records are returning null (Even the record with Id exists on ES index).

I need some help for handling concurrent requests with fast processing and less failures.

Note : Id is not null or empty in any of the message.