plastic pipe fitting failed - why?

[Jennie]

I came across one of these fittings last week, that had popped under the bath.

http://www.NoLinkingToThis/p/jg-speedfit-pem062215wp-straight-reducers-22mm-x-15mm/18873?kpid=18873&cm_mmc=Google-_-Product+Listing+Ads-_-Sales+Tracking-_-sales+tracking+url&gclid=Cj0KEQiAifvEBRCVx5up6Ojgr5oBEiQALHw1TnyM6UvfOf2THe3qlGiMyp3DnnUV982H9oo1GpfGLlUaAkN28P8HAQ

It had connected the hot feed plastic pipe to the flexi 22mm tap connector. It was the compression flexi bath tap connector that popped off the thick, rigid 'long' bit of the fitting. Luckily the tenants were in to turn off the water.
I remade the joint, with a new (copper) olive, and PTFE tape, pipe inserted fully into flexi compression, all tightened up, etc. No leaks or weeping. All seemed fine.
Apparently it held for 36 hours, and then popped again. (Thankfully the tenants were home to turn off water again).
I'm going to return next week, and replace the fitting (probably with a short section of copper pipe, going into a brass compression fitting with insert for the 15mm plastic pipe).
I'm not impressed the joint blew after I'd remade it.
The fitting was used as designed, and had held for several years before failing. But I can't help thinking this fitting will never be as good as copper pipe. An olive on copper pipe can bite in, as the metal has malleability. On plastic pipe, isn't it more prone to sliding off? Especially if the plastic pipe expands/contracts over time (it expands more than metal) and ultimately becomes less tight?

 

[kozak1968]

It might be a bad thread on the flexi which when you tighten up, it feels tight.

 

[joni os]

Always prefer SS inserts. Never use insert with O ring seal on compression fittings. Copper olive for plastic pipe. If all this is done then failure causes concern. Was there thread still ahead of nut when fitting tightened. I've experienced an increased frequency of undersized olives which allowing nut to bottom out on thread.