Help my 2 year old Doxie pees on the couch?

I have a mini doxie who is 2 years old and semi-litter trained...but she still has accidents on our brand new couch and I am not sure what to do. I have her on a schedule and she will do her Business in the proper place when I take her to the potty place but will not go there on her own and will not give me any indication that she needs to go potty. We also have an 11 year old boston terrier in the house whom is fully litter trained and will do her Business all by herself with out anyone have to take her so I can not understand why my doxie will not and I am nearing my last straw with her...is it still possible to train her even though she is 2? She has also been checked out for bladder infections and does not have any...please help