housebreaking shithzu

Question
Our Shithzu (sp?) is now 5 months old.  She goes potty outside when I take her on a leash but if I miss her timing by a few minutes she will pee inside wherever he is, on the couch, floor, bed, cage etc.  She doesn't come and tell me she has to go out.  We have put a bell on the back door but that hasn't worked.  She is near me most of the time so I try to take her out at least every 45 minutes. Sometines it has nothing to do with timing either.  She may go outside and 15 minute later go inside.
Any help would be wonderful.
Thanks, Sara

Answer
First always check to make sure there is no urinary infections,  Then I would try to teach her to hold it.  She won't ask right away until she recognizes the urge to go.  Try a crate.  Keeping her in there with water and food for a couple of hours at a time.  Then take her out.  When she is back in put her on a leash and hook it to your belt so that if she has to go you will be right there to corrct and bring her out.  Once outside, reward her.
Nancy