Hello, I have a bichon who is 4 months old. I also have the problem that although she will go outside I have to make sure she is outside when she needs to be. I have taken her for walks at times and she has peed on her return so this is what i did..... In the very beginning i got a diary and put down when she fed and then when she peed and pooed - this is helpful as you then have some idea about her body and how long it takes her to go to the toilet. With my puppy I find that about 2 hours after eating she will poo. She will poo and pee in the morning although it can take up to half an hour for her to do her morning poo which is sometimes done in the house if i cant watch her in the garden (the wee comes pretty quickly). She is also crate trained but it might be an idea to completely clean and flush out any smells which she associates with going to the toilet or using a smaller crate - sometimes if they are too big there is enough room to pee poo and sleep!
I am not saying that i have solved the problem and I am now investing in some 'poochie bells' you can order them on line, they have a website for more information. The point of them is to get you dog to ring them when they want to go out to the toilet. Mind you in your case you need to make sure she is going in the garden. When I have calculated times, waited for a poo and been rewarded, as the dog is going i have a word to associate - my word is 'busy busy' and she gets lots of praise and treats - rather like a doggie jackpot! You can also you a clicker to mark the correct behaviour which goes after the word. I hope that eventually she will go on command but i do get the idea that at least she knows what i am asking her to do. I hope that some of my ramblings help. I have also been told that for some dogs it can take months to house train and some bichons can be stubborn in this area. Patience, perserverance and i have to say that puppy training classes helped also.