Couple of things (well, perhaps some more):-
1. Was registered parcel sent by airmail or surface mail? I see that these days even letters to the UK must have an airmail sticker if going by airmail.
2.Parcels posted to countries outside of Ireland and even to an EU country would be subject to Customs check and possible delay; especially to the Netherlands.
3. Any post item is deliverable to an address only; not to a particular person. It is the same with registered items. So is your parcel somewhere at the address awaiting pick-up?
4. Postal companies have some difficulty in delivering to some addresses e.g. an apartment block with locked exterior gates and locked apartment complex doors to buildings. Sometimes there is no exterior letter box of any description.
5. Occasionally only notes regarding registered parcels are delivered asking addressee to collect at local post office.
6. There are addresses that sometimes cannot have a delivery i.e. ongoing non attendance.
7. We are just after a bank holiday weekend, so quite a few non working days (just pointing out the fact).
I hope this is of help.