The update of the Service Config Item Association table for Dynamic CI Groups is contained within Business Rules in each of the related lists on the CMDB Group record that is referenced within the Dynamic CI Group record. This uses a private script include so you will not be able to see how it is updating, nor track the progress of any updates. I would first check whether inside the CMDB Group you can see all the CIs you are expecting based on the queries defined. I would also examine the queries to ensure you do not have overlapping conditions (e.g. one query on a parent CI class and another on one of it's child classes) as I have seen confused or delayed results. 

However I will say that I have usually seen updates inside a minute, even when initially creating tens of thousands of Service Config Item Assoc records, and have never not seen CIs, unless the DCG limit of 10,000 has been reached. If you have done all of this and still do not have answers then it needs ServiceNow Support to investigate.