Even brand new seals leak if you're unlucky.
When we had our van being examined for leaking slider windows at the dealers they loaned us a brand new Kombi. That night it was raining, so out of curiosity I went out and had a look. Both windows were leaking!
The dealer eventually replaced our window which has been leak free for a number of years.
However, when the warranty ran out the other side started to leak. After reading all posts on the forum and on Google I found a fix myself and it has worked for me.
Very carefully, remove the sliding window and place it somewhere safe. Thoroughly clean all seals with soapy water. Ensure all grit etc. is removed. Do this for the removed window as well. Using a very thin piece of plastic, e.g., a cable tie or similar clear the debris out of the 2 drainage slots. Don't use anything metallic as it could scratch the bodywork below. Using a water filled syringe with a tube fitted on the end purge the circular drain holes until they are clear.
Then, carefully replace the sliding window.
If you park the van nose down any water ingress will allow the drain holes to do their job. Nose up defies physics as water doesn't travel uphill.
It also helps if you never open the sliders!!!!
I hope this helps.