When you send an SMS out of Assignar and it says in the outbox the status says "Delivered" this is because your worker's telco carrier has shot a message back to our system saying it is being delivered. What happens beyond their provider saying it's been delivered is out of our control.
There are a few factors why this is happening:
They could be out of phone reception when you send the SMS and therefore it is not received.
Certain carriers will blacklist numbers that are sending out high volumes of SMS, we have seen this happen in the past with other customers who are high users of the SMS feature. Telstra, Optus, Vodafone etc block numbers and the worker needs to call them to have it unblocked (however I don't think this is the case for you)
The carrier of your worker could be experiencing high volumes of SMS and that creates it to be put into a queue and delivered at a later point. As it keeps happening with a few of the same users, I have put in a request to investigate with our carrier between your SMS number and their number (specifically Stuart) to see if they are getting any further information that could help us.