When I stay in a US hotel, I like to leave a tip for housekeeping. I usually leave $2 (two singles) somewhere obvious that this is for them - not just something I've left behind by accident. Usually by placing it on an otherwise untouched surface, like on the TV unit or next to the drinks cabinet near the front door. Sometimes on a pillow, but I'm not a fan of this because sometimes it falls off the pillow and I find it on the floor or under the bed later on.
However I've noticed that often (I didn't measure it, but I'm going to say 1/3 of the time) housekeeping does not take the tip.
Is there any reason why not? Am I not leaving it in an appropriate place? Do they think I left the room tidy enough that it doesn't warrant a tip? Do some housekeeping people just not accept tips?
Mostly I'm concerned that my ettiqute about where I'm leaving it is incorrect.