From the context you provided, it looks like you might be using unary pulling for your subscribers.  In general, we recommend using the high-level client library instead.

With unary pull, Cloud Pub/Sub may return fewer messages than the maxMessages value you specify in your requests. You can verify if your requests are pulling the maximum number of messages by comparing the Sent message count and the Pull request count metrics. You should also make sure that you are not setting returnImmediately to True.