Kitchen sink pipes underneath leaking

[adgo1]

I removed three of these sections to clean and when I put them back together I noticed leaking.
Please can you advise on how to attach them correctly.
Thank you.

 

[Basher]

Undo the joints that are leaking and check the black washers are on tightly, correctly seated with no twists, and that they are not loose, damaged or dented (if they are, new ones can be bought cheaply). Then do them up again tightly!

 

[adgo1]

I put it back together but water is still draining away slowly. I tried drain unblocker and the pipes are clear.

 

[Basher]

It shouldn't leak even if it's full of water, so I think you have two issues - the pipe joints, and also there's likely some degree of blockage further down the pipe, or in the sewer it feeds in to.
Have you tried a sink plunger?
Maybe time to call in a professional.