Stihl HT133 pole saw problems

[RL62]

Being having problems with this reasonably new saw. The pole keeps disconnecting from the saw end. Have traced the problem to the Circlip that should stop the inter pole from sliding down the outer. Clip keeps getting pushed out of place but cant work out why and cant move it back without plies so should be holding well. See pic. Any ideas? Have seen a post about making the groove that holds the clip deeper but seems an odd thing to need on a new machine ! Design fault as I see this has now been discontinued?