I had two similar experiences with EMS, the first small parcel took almost 6 weeks to arrive, it went off the tracking radar at BKK with no way of findingoutwhathadhappened.
The second, important legal documents, took 4 weeks again, they disappeared from tracking at BKK.
Due to the urgent importance, I had to reprint the documents and have them legalised again, then sent via DHL. These arrived 4 days later.