It seems that packages shiped as "China Post Ordinary Small Packet Plus" don't get tracked.
Example:
From AliExpress with trackingno: 04713489856
Shows up in AliExpress internal tracker (data from cainiao [1]) but not in the app or 17track [2].
/jp
[1] http://global.cainiao.com/detail.htm?mailNoList=04713489856
[2] http://www.17track.net/en/track?nums=04713489856