1. This is the tracking of your parcel:
China Post Register Air Mail Tracking System
Parcel No.RK520538985CN tracking result
2016-09-20 17:11:46.0 张家港city 广告分公司 received
2016-09-20 18:18:16.0 张家港city 广告分公司 customs scan
2016-09-20 19:17:15.0 leaving 张家港city 广告分公司 ，next station 张家港
2016-09-24 07:39:14.0 Suzhou Transit Station export security scan
2016-09-24 09:13:15.0 Suzhou Transit Station export customs scan (domestic transit )
2016-09-25 18:36:32.0 Shanghai Transit Station export security scan
2016-09-25 21:20:35.0 Shanghai Transit Station export customs cleared
2016-09-27 12:10:00.0 arrive Shanghai Int'l Airport
2016-09-27 12:26:45.0 leaving Shanghai Int'l Airport going to next airport/seaport

2. I also found this:
International Mail# RK520538985CN Tracking Result
Summary
Tracking number Origin Destination Status
RK520538985CN CHINA NETHERLANDS On route

History Detail
Local date/time Activity Location Remarks
20-Sep-2016 17:11 Received by Post Office 21566400
26-Sep-2016 05:33 Departure export customs SHANGHAI

3. According to the above tracking your parcel was mail on the 20 Sep, normally parcel to European country are taking 3 week. But this time it seems that Netherlands is a bit longer. Apparently China Post had delay shipping to Netherlands we don't know the reason.

4. According to the tracking your parcel is fine and you are the only one that can track it on the Post NL site because we can't. You could also phone Post NL and try to verify the info about parcels getting delay from China.

Sorry I could not help you more.