Your first thermal cycle with imm silver should be the easiest to solder. That you see copper indicates you did no soldering, got no wetting, produced no intermetallics, etc. Tell us again which components did not solder? Wuzzis "plug" thing that yer talking about?
Assuming you are storing and handling your imm silver boards properly, maybe your board fabricator: * Let your boards sit too long between microetch and immersion, allowing corrosion of the copper surface of your pads. * Plated too thin of an imm coating, allowing corrosion of the copper surface of your pads. * Legend printed after imm silver. Since chemicals or heat used in screening can cause tarnish, staining and unsolders, unless they are using a UV ink.
Having said all of that, it makes us very nervous if only a few components are "popping". If that's the case, yer going to have to start talking about your processes, materials, handling, etc.