[boost] Bjam stops

Hi to all, I'm running bjam 3.1.10 after building and installing the Boost library, I want to rebuild a single library (e.g. serialisation, or whatever: it's the same), if I type bjam -qad2 -sTOOLS=gcc --with-serialisation Jam starts, parses its configuration files and after about 20 seconds writes "found [n] targets..." where n is a number (e.g. 342) and then stops instead of beginning the new build as usually does, what's my mistake? Thanks in advance, Gianluca