In Regard to the House training Issue
If your were not taking your Boxer outside enough we would assume that was the problem. However, since you are taking her outside and offering plenty of opportunity to urinate in the appropriate area, yet she still pees inside, we do suspect another issue.
This may be a health issue
. When you look at pets that have accidents,14% of those dogs have a urinary tract infection that is triggering the problem. And female dogs develop this more often than male dogs.
We highly suggest bringing your dog to the veterinarian for a complete checkup and let the vet know what has been happening. If there is indeed a urinary tract infection, the use of both antibiotics and antimicrobial medication should fix the problem.
This may also be a marking issue. Some people falsely assume that only male dogs mark. However, female dogs mark also. Marking is the behavior
a dog will display to "let everyone know" what they consider to be their territory.