Valcyte has potential severe side effects, the worst being neutropenia and liver problems. Both are slow to develop, so careful blood monitoring (with basic lab tests) allows you to avoid them because you can reduce dosage or quit the medication as soon as you see signs of either problem developing.
Unfortunately most doctors, including some ME/CFS specialists, are afraid to prescribe Valcyte because of those potential serious side effects. If they prescribe 3 months worth of Valcyte and you don't come in for the lab tests every month, you could become severely ill. They don't want to be responsible for that. Or more likely, they don't want to risk being sued over that.
Dr Lerner will often only prescribe enough Valcyte to get you to your next lab. So, no labs, no Valcyte. However, that means he has to see you, evaluate your labs, and write a new prescription every 4-6 weeks. Many doctors are not willing to put that much effort into each patient, so they just flat refuse to prescribe Valcyte at all.