How can metastatic breast cancer progress?

Metatstatic: Metastatic means cancer cells spreading or moving to other sites. Breast cancer cells can spread via lymphatics to lymph nodes. Breast cancer cells can spread via the blood stream to other sites such as bone, lung, liver, etc.

It can and it does: Progress of cancer means the cancer is getting worse. This is not uncommon as the cancers tend to spead further in the body and tumors get larger in size especially when the treatment is not working. Cancer is either regressing(with good treatment) or progressing(without treatment or with the ineffective treatment).