What is the specification of the hardware your cluster and ingestion pipeline is running on? How much data are you trying to ingest per day?
There are a number of factors that can limit performance and cause the ingestion pipeline to not be able to keep up with the flow of data. Indexing into Elasticsearch can be quite CPU intensive and also cause high disk I/O load due to the constant merging of segments that take place. Make sure that you have followed the guidelines described here and then monitor your cluster to see if there is any apparent bottlenecks. If there is nothing that stands out, try adding additional load to see if there is spare capacity. If Elasticsearch is the bottleneck, scale up or out depending on what you current cluster looks like.
If Elasticsearch has spare indexing capacity, it may be the ingest pipeline that is limiting throughput, as these type of components can be very CPU intensive. Monitor the components of the ingest pipeline and tune it if possible. Try scaling up or out depending on what your pipeline looks like.