You need to reselect the care for self action semi-regularly for them.
Bienchen ran through exactly how the game handles taking care of sims left at home. FIrstly, it does not actually simulate what they are doing. The cliff note version is pretty much that care for self slowly fills one need at a time, whichever is lowest when you select the action. And it does NOT switch to another one until that one is completely full. This means some other needs may likely plummet into red while the one need is still being filled.
By reselecting the care for self action, you get the game to switch to whichever need is lowest rather than continuing to fill whatever one it was working on.
Additionally, sometimes it doesn't start doing the care actions at all unless you select it....even if it indicates that is what the sim is doing. So make sure you always toggle the action right away as well, when leaving sims alone. The game itself will have them doing literally nothing at all sometimes if you don't.
Edit: wish I could find the original thread bienchen posted in. She explained it really well and has written some away-action mods, so obviously worked through the EA scripts for the process and understand exactly what the game is doing. I think I summarized it pretty accurately though.
