1. This is the full tracking of your parcel:
China Post Register Air Mail Tracking System

Parcel No.CP308330955CN tracking result
2016-03-23 13:08:15.0 建内大街post dept. received
2016-03-23 14:33:40.0 建内大街post dept. customs scan
2016-03-23 17:55:43.0 leaving 建内大街post dept. ，next station 京大平面
2016-03-24 09:03:19.0 Beijing Transit Station export security scan
2016-03-24 13:00:54.0 Beijing Transit Station export customs scan (domestic transit )
2016-03-25 10:13:41.0 Guangzhou Transit Station export security scan
2016-03-25 13:50:25.0 Guangzhou Transit Station export customs cleared
2016-03-30 13:15:00.0 Guangzhou Transit Station import customs cleared (domestic transit )
2016-04-03 09:07:27.0 Beijing Transit Station import customs scan
2016-04-03 11:18:25.0 Beijing Transit Station import customs cleared (domestic transit )
2016-04-05 08:14:24.0 arrive 建内大街 Delivery Office delivery office
2016-04-05 08:16:05.0 建内大街 Delivery Office attempt delivery
2016-04-05 11:00:00.0 建内大街 Delivery Office delivered
2016-04-22 09:23:45.0 arrive China Post Group Beijing city 东城区建内大街post dept. delivery office
2016-04-22 09:24:45.0 China Post Group Beijing city 东城区建内大街post dept. delivered

2. Your parcel was sent back to the sender of the parcel. You should contact sender of the parcel as soon as possible.

3. Here is were the problem started:
2016-03-25 13:50:25.0 Guangzhou Transit Station export customs cleared
2016-03-30 13:15:00.0 Guangzhou Transit Station import customs cleared (domestic transit )

4. Either on the 25 March the parcel from Guangzhou got to Singapore and was sent back on the 30 March to Guangzhou. Or on the 25 March the parcel was suppose to depart from Guangzhou but for unknown reason did not and imported back from Guangzhou on the 30 March.

Hope this help