Repeatedly not paying for oncology related tests

My Mother is receiving prolonged treatment for Breast cancer. Her Oncology treatment requires routine blood tests to guide the treatment strategy. We have had repeated instances whereby Momentum Health suddenly stops paying for certain of the tumour marker blood tests that are necessary for oncology (normally August/September of each year based upon the history). This is the 3rd year in a row that I am dealing with this same issue. I then need to spend months trying to extract sufficient detail to resolve the issue, deal with debt collectors from the Pathology lab etc. The Oncologist claims that their treatment plan specifies the tests and Momentum must think not - neither organisation is willing to give me specific details so that I can get to the bottom of the problem. I logged an assistance request via the regular Momentum help line, received a response requesting more detail of the issue and have requested updates repeatedly, but have not received any response. Somehow, the root cause of this issue is not being resolved and I am forced to run this gauntlet every year in addition to supporting my deteriorating Mother - the stress is considerable. Please can someone in Momentum look into this issue and solve the root cause permanently and not just the symptom of the issue?