The work in progress metric is updated using an acknowledgement delivered by facilitators. This messages allows to confirm the delivery to recipient and change the message status in statistics.
When facilitators are overloaded, those acknowledgements are sent with some hours of delay to Splio. This latency sounds like an issue and some users can understand that messages are not sent or delivered with a delay.
In facts, all messages are sent in timely manner even if statistics say otherwize because Splio is designed to send messages firstly and update work in progress metrics afterwards.