Intermittent message delivery via sendContent API (HTTP 200 but user doesn’t receive message)

Post content:

Hi everyone,
I’m running into an intermittent delivery issue with ManyChat API and would really appreciate some insights from the community or the ManyChat team.

Setup

• Platform: Facebook Messenger

• API used: POST /fb/sending/sendContent

• Message type: Image + text + quick reply

• API is called from n8n

• HTTP response from ManyChat: success / 200 OK

• Subscriber ID is valid and active

The issue

• In the morning, everything works perfectly:

  • Flow runs smoothly

  • API sends messages successfully

  • Users receive image + text as expected

• However, in the evening, delivery becomes unreliable:

  • Sometimes the message is delivered

  • Sometimes it is NOT delivered at all

  • n8n still reports the request as successful

  • No error response from ManyChat API

This happens even though:

• Subscribers are active

• They have recently interacted

• Message volume is very low (definitely not near 5 requests/minute)

• Same payload structure is used every time

Additional context

• The workflow before sending the message includes AI image generation, so the delay before calling sendContent is around ~2 minutes

• I’m located in Vietnam (GMT+7), so the issue mostly happens during local evening hours

• I suspect this might be related to:

  • Traffic-based throttling

  • Hidden rate limits

  • Messenger delivery window behavior

  • Or regional load differences

My questions

1. Are there unstated rate limits or traffic-based throttles for sendContent?

2. Can sendContent return HTTP 200 even if the message is silently dropped?

3. Does a long delay (~2 minutes) between user interaction and API call affect delivery reliability?

4. Is there any best practice to ensure reliable delivery for image + text messages via API?

Any insight or confirmation from the ManyChat team would be extremely helpful.
Thank you in advance!