When does a duplicant go by a wash basin and not wash their hands?

Most likely the cause is that some other dupe is already using the sink/wash-basin. You should put in one sink per every toilet and set them all to be used only on one direction. That should solve your problem.

I found that if the toilet is to the right, and the basin is right next to it to the left, they just ignore the basin completely. So I solved it two ways: one is to put the toilet to the left, and the basin to the right, or to have some space between the two.

I know this is a bit of an older thread, but I just had this problem, and I was trying for a while to get my dupe to wash his hands when passing by the wash basin. I tried making it bi-directional, I tried "Move to" several times, and nothing. I was just about to restart the game when I noticed something, the basin tubes on the side - where you can see the blue (clean) and green (dirty) water, they were empty.

Should look like this:

But instead looked like this:

So clicking on the properties of the basin, I looked at the "Contents" and found this:

"Contents None"

I then realized that I had a priority of 5 on the basins, so they hadn't added water to them. I upped it to 9, and immediately someone went and got water, now I see this:

And then my dupe immediately washed his hands as he passed by the basin. It didn't say it needed any materials (much like a research station says it's waiting on water or dirt, or an oxydizer waits on algae) so I didn't realize they were dry.

Hope that helps.