Oracle data pump import parallel configuration
Two common reasons:


Each worker thread uses a seperate dumpfile and requires exclusive access to it. If you have fewer dumpfiles than the degree of parallelism you provided, the parallel threads will not be fully utilized. If you have a single dump file, only 1 worker thread can be active. 
Tables that contain Basicfile LOB columns are not be exported or imported in parallel. This is a known restriction and documented in: DataPump Export/Import Of LOBs Are Not Executed in Parallel (Doc ID 1467662.1)


Workarounds include:


seperate partitions can be imported in parallel 
start multiple data pump jobs and distribute data between them using the QUERYparameter.