can anyone help? we've had our dog for 4 mo. rescued her from shelter. she is 18 mos. old. she doesn't want to go to the bathroom in our yard! is holding out to be walked. i walk her each night but need to train her to go in the yard. she has held it as long as 18 hrs!! most days she'll go (reluctantly) in a.m. then will hold it until our walk at night -- usually around 9:30 p.m. any suggestions??