I have recently had a lot of troubles with 2 PCB assy's which had been working in products for 10 years or more, but about a year ago started having problems. The symptom appeared to be leakage in the preamp, as these boards have high gain and also modulated input signals of a few dozen Hz. The problems began when we started using different suppliers, such as a popular low-cost one in Queensland. I could not pin down a correlation with a particular supplier, but all the bad boards used Chinese blanks. However, we had also used Chinese boards for some years without problems. That is what made it hard to simply blame chinese suppliers.
Circumstantial evidence points to US origin FR4 material being much better than the Chinese. Perhaps Chinese makers were importing US FR4 for some time but now make their own, but that is conjecture. When we switched to a ceramic fibreglass composite by a particular maker, the problem disappeared. Also, as a test, we tried some PCB's from a large reputable (but pricey) Melbourne PCB maker. They were good too, but if Chinese FR4 is a problem and they start to use it, then we would still have a problem.
I can't say what the nature of leakage is, but the ceramic PCB's do work, with a few $ extra per board. I will say that we have about 150 other board types with no problems with Chinese suppliers, but they are not so sensitive to leakage or dielectric variation.
BTW, we tried baking, guard rings etc.