Cannot get compression fitting to seal

Section62

grumpycrab said:

Hello. Inserting  a hard plastic pipe with olive and nut (15mm) but I cannot get a leak free seal. I cannot work out whether the olive is too loose on the hard plastic pipe or whether there's not enough thread on the pipe I'm fitting it too.

The plastic needs to be soft enough for the olive to 'bite'.  If you are using some kind of hard plastic fitting rather than pliable pipe this could be part of the problem.  Ditto the use of copper olives as per shiraz99's comments.

Also, the pipe does need to be inserted sufficiently into the fitting.  If you are using pipe inserts then the additional length of these can leave too little pipe inserted into the fitting to get a good (and strong) joint.  The 'superseal' type inserts can be particularly problematic with fittings with inadequate insertion depth.

Apjs87

As said, this is more than likely down to the plastic pipe missing an insert. Without that, the pipe will be too soft.