1/2 inch to 15mm Shower Mixer Leak

[Plastered]

I need a bit of advice and encouragement. I am setting up a shower mixer which had hot and cold in, and supplies two showers. Each of the four connections is 1/2 inch female thread, so I have four 1/2 inch to 15mm connectors screwed in. 3 of the 4 are fine, but I cannot get the 4th to seal under pressure. There is always a small bead of water appearing at the face of the 1/2 inch connection after a few minutes (the 15mm end is ok).
I have tried using a few turns of ptfe tape, and 5 or 6 turns. Using 5 or 6 turns makes it slightly worse.
I have changed the 1/2 - 15mm which makes no difference.
Any advice on how to seal it completely.
Is there any drawback in using an O-ring on the face in addition to the ptfe tape.

 

[PlumbBobBob]

5-6 turns of ptfe isn't enough imho go up to between 10 and 15 and see how that seals

 

[Plastered]

After a call to the manufacturers technical helpline, it turns out that there have been a few cases of manufacturing problems with this unit which manifests itself as a hair line crack.
I am now waiting for a replacement to see if this cures it. So it may not be my own plumbing incompetence after all.

 

[AdamMcC5]

5-6 turns of ptfe isn't enough imho go up to between 10 and 15 and see how that seals

totally agree with this

 

[SimonG]

Don't forget the magic paste.