I am having this problem too, and it is getting to be a problem.
I have eight Sims (two adults, six kids ... long story, don't ask) in a large house on a 40 by 40 lot. The house has a basement, a ground level and a second story. If I do not watch my Sims do what I ask them to do, they will just stand there and continue to have their Needs decay. This has resulted in numerous threats of the social worker taking my kids away, several instances of ALL my Sims peeing themselves or letting their Fun and Social needs decay into the red WHILE THEY ARE QUEUED TO TALK TO EACHOTHER WHILE PLAYING A VIDEO GAME, and the adults have passed out from exhaustion while standing two feet from their bed after I asked them to trim their Bonsai Tree (which never got trimmed and has since died).
I thought this was because I had too many Sims in the house. So I saved the game and went to play one of my Single Sims games (different world, different save file). Sure enough. If I ask him to take a bath in the second floor bathtub THEN go downstairs to queue up him making some lunch then tending the garden, he will NOT use the bathtub. He will stand there looking dumb-founded at the tub (Hoji, sweetie, I know you are a surfer but you also have maxed out logic, running water should not confuse you) until I go back up stairs and watch him.
I swear, my Sims are like my pre-school aged nephew; unless I watch him do something, he won't do it. If I wanted to babysit, I would tell his mother to drop him off at my house more often; at least he knows how a tub works.
