America is quiet on Syria because of Obama's policies towards Iran and Russia. . Obama wants to avoid a war with the former and do a 'reset' with the latter. Since one of the main reasons the Iranian are building a nuke is in order to avoid 'regime change' (say if they fire on their own people), Obama acts as if the less dangerous Syrian WMD arsenal is very deterring ("see! you don't need nukes to deter us!"). If Obama acted on Syria, the Iranian Mullahs may feel they need a nuke, so Syria is being sacrificed to keep them calm.
EU's Tusk warns Turkey to cut refugee flow (Reuters)
from the article: America has given up on Syria