Leaking under sink

[DIY67]

I have absolutely no idea how this is happening but our newly installed kitchen has started to leak under the sink after 2 months. I have unscrewed and re-screwed the connection several times with the black rubber seal in contact fully and yet when screwed on it starts dripping from the left side, and yet this is a new piece and no obscuration in the diameter of the pipe as I have thoroughly checked. So why oh why is this leaking ?

 

[Elsa TLC]

More detailed pictures of the whole thing

 

[Elsa TLC]

Looks like the drip is from above!

 

[Alec_t]

Looks like this part is mis-aligned/cross-threaded ....

 

[DIY67]

Looks like this part is mis-aligned/cross-threaded ....
View attachment 349620

I've just checked and it's def leaking from the lower connection. I think that water was a drip from splash trying to clean up. It's weeping out all the time while my hand feels the above which is bone dry. I just can't fathom why it is leaking. Even tightening it up more doesn't help.

 

[Elkato531]

Possibly slightly mis-aligned. slacken off all the other joints in the vicinity then make sure the suspect joint is properly aligned and tighten that first. Firm hand tight should be enough. Tighten all other joints and test.

 

[Justin Passing]

It looks crooked.
What's that bit?

 

[DIY67]

Ok, I disconnected and reconnected all parts and even swapped the u bend round and it still leaked from the left side of the left U. There seemed to be some minor defect in the thread of the left bend but even swapping that again gave the leak on the left side, and a little on the right. I tightened them both up with slip pliers and so far no leaks. I was going to introduce ptfe tape but it seems it isn't needed. The only difference is the leaking joint won't tighten as much on the left as the right side.