not cleaning herself

okay, before i left on vacation, i would have to clean sara's behind every once in a while because after she went poop she didnt clean it... well i got her back today and there is a little bit of dried poop on her bum which tells me she is still not cleaning herself.... she is over 5 months old.... when does this self-cleaning thing begin... i cant follow her around and wipe her bum for her everytime she goes...
is there anything i can do to encourage her to clean herself???
just wondering, thanks!

Some cats I think just dont I dont know of any ways to encourage them either.

Zoey's fat pad has gotten a bit big so she has a problem reaching all the way around there and I will have to clean it sometimes. Saki never cleans his bum.. but he doesnt have the problem of anything getting stuck there usually.