Pub/Sub shows all JSON messages published but the real-time dashboard built with Cloud Dataflow is missing some messages. What should you do next?

The most effective next step is to run a fixed, known dataset through the Cloud Dataflow pipeline and inspect its output. This isolates the problem to the Dataflow pipeline itself. If the known messages are missing or transformed incorrectly, the issue lies within the pipeline logic. If the known messages appear correctly, the problem is likely upstream (Pub/Sub publishing or data ingestion) or downstream (dashboard display). Inspecting the dashboard application for display errors is a good later step, but first, confirm Dataflow is processing data correctly. Using Stackdriver Monitoring on Cloud Pub/Sub can confirm messages are published, but it doesn't tell you if Dataflow is receiving or processing them correctly. Changing Dataflow to pull from Pub/Sub is a configuration change that might not address the root cause and could introduce new issues; it's not a diagnostic step.