What are the main reasons of container(s) demurrage and detention or both combined demurrage and detention ?

In very brief, the following are some (not all) main reasons for that are:

1- Delays due to incorrect documentation.

2- Delays due to late of receipt related documentation.

3- Delays due to documentation lost in transit.

4- Delays due to release procedures of cargo at destination port.

5- Delays due to customers clearance or cargo inspection.

6- Delays due to receiver being unreachable or miscontacted.