bending insert help

[cdawg2016]

my issue

I bought the petg rigid tubing from primochill size is 3/8" id 1/2 od  and I bought a bending kit from alphacool for my size tube the tube insert would not fit into the tube no matter what I did I heated,froze,stretched,sanded everything nothing worked so I ordered a tube insert from primochill same company as my tubes and the same issue it wouldn't fit I sanded it slightlyand I  boiled,froze,stretched  and nothing the white insert in the pics below is from alphacool and the black is primochill

 

question

 

what am I doing wrong?

I seen people slide theirs in the tubes no issue what cant I do to get the insert to fit?

 

[tiberiusmoon5]

Wetsand the tube insert with 400 grit and make sure the tube insert is also wet while sanding.