UPVC Window leaking

[DIYinBerks]

We have a couple of windows in a conservatory that leak regularly in heavy rain, just two units out of many. The water leaks in near one mitre in each case. The windows are of the type where the sealed unit can be taken out from the inside, plastic bars with rubber wipers provide the seal.
At first I thought it was blocked drainage channels so I have had the sealed units out and cleaned out all the muck: water seems to drain out nicely, But when it rains, water still drips in - very gradually but before long there is a good size puddle on the floor. Any suggestions what to look for or what to try next?

 

[crank39]

Is there a crusty residue seeping out of the bead where is touches the frame?

Was there any water however little sat in the frame under the dgu?

Has the gasket (against the glass) on the outside shrunk and pulled away from the corners?

 

[DIYinBerks]

No, no residue that I can see - but I did clean everything when I took it all apart originally and there might have been residue then. Taken it all apart 3 times now including check today.
Yes, definitely some water sat in the frame under the DGU. I can't see how/why it makes its ways over the ridges though. Seems as if it prefers to make its way inside, up and over the ridges in the frame, rather than drain out the (now cleared) drain holes.
Yes, it doesn't meet that well in the corners. But shouldn't the drain holes allow any water that get in past the gasket drain back outside again?

 

[crank39]

Water ingress and egress is a funny thing, has me baffled most days when i'm out inspecting lol!

Residue around the bottom bead is a clear indicator of water sitting there and not draining, again how it builds up to clear the ridges instead of draining is a mystery but you've found water under the dgu so it proves it ain't draining.

As for the gasket then either replace or make the gap up with a smudge of black silicone, can be done with the glass in situ

 

[DIYinBerks]

Looking closely at the outside gaskets the mitres weren't cut that well and so there are quite some gaps.
Next challenge is waitiung for some dry weather so I can try mastic...