Silver amalgam fillings predominantly contain silver a known bactericidal agent and mercury which a known toxin and has bactericidal property. So how is it that the plaque bacteria survive near the filling margins and produce fresh caries?
Amalgam fillings are made from an alloy. The properties of the chemical elements bound into an alloy are different than the properties of those elements in different forms. The fillings do not release any biologically significant form or biologically significant amount of either silver or mercury. They are essentially inert.
Plus silver antibacterial effects are quite modest. The existence of fine cracks and gaps between amalgam and tooth provide wonderful hiding places for bacteria - and that shelter is probably more important to the bacteria than any negative effects of the silver.
The silver and mercury are heavy metals and absolutely poisonous. However, they are present as amalgam in fillinmgs - an alloy with various constituents including mercury and silver. As such, mercury is not as volatile as it is in its pure elementary form and is basically bound to the rest of the solid-state metals. However, it can still evaporate in small amounts nonetheless, and I quote from the website of the FDA on this material:
Hence, although mercury is released, it is shown to be dangerous only after it accumulates in the organs. It, apparently, does not linger around at the place of release. Silver likely stays where it is in the amalgam, as it is a solid. Given that the FDA does not make mention of it, and since the FDA has done a thorough study to the adverse effects of amalgam, silver is probably not released in any significant amount.
In all, I think the explanation to why cavities still form is because the mercury vaporizes and re-accumulates elsewhere in the body into, possibly, toxic amounts. Other metals, including silver, are not released at all.