Check the province culture of the province on which Silesia has a core. If the culture of that province is schlesian (Silesia's primary culture), then you should be able to release Silesia. If the province contains any other culture, then no. At the start, only Breslau and and Ratibor have schlesian culture, so if these provinces have been czechisized or germanized -- and schesian hasn't spread to other provinces -- then you won't be able to release Silesia (regardless of the existence of a core).
If the culture does match the province you are trying to release, see if Silesia already exists somewhere else. Perhaps some random slavic province has rebelled to it, or maybe they have a lonely colony in the New World. You should be able to see them in the ledger, unless they're outside the regions you have mapped.