I had that trouble while playing this weekend. I have 8 poor sims on the lot in Sulani that has the crashed ship on it. I removed the furniture and bathroom from the basement under the ship and scattered 4 pee bushes around the lot. Several of my sims never made it to the bush. I understand them being fussy if the bush isn't, uh, fresh, but even when I sent them to the most distant bush that hadn't been used, they still would stop and stare around...next thing you know, puddle time.
I saw the routing issue happen with Mortimer Goth. I sent him to a bush and he walked way out to the side, turned a sharp corner, and marched in the direction of the bush. Then he just kinda wandered a bit till finally winding up at the bush, which, by that time, was occupied. He drifted away while I was doing something else and left a puddle. I noticed that Eliza Pancakes was constantly needing a shower, now I'm wondering if she was having bladder troubles, too.