Are you calling setProcessingPrecision? This method is called by the host, see the docs:
A client of the AudioProcessor calls this function to indicate which version of processBlock (single or double precision) it intends to call.
A client of the AudioProcessor is a DAW (i.e. plugin host), using the extra functionality that the AudioProcessor serves.
Your plugin offers double precision processing by returning true from AudioProcessor::supportsDoublePrecisionProcessing(). But you will have to select in the host, that you want to do double precision processing in the plugins.