I could be wrong, but I think that symbols generally share the same canvas properties as the documents in which they're created. What sounds odd is that your symbol is taking up extra space on the document canvas and preventing you from selecting items beneath it. There must be something occupying that space within the symbol, right? For example, is there a masked object in the symbol? (Objects that are masked remain selectable beyond the mask boundaries.) Could there be some invisible element at the edge(s) of the symbol canvas—for example, a vector with no stroke or fill?
I'm a little stumped. Do you have a screenshot for this?
One more idea: Try using the Select Behind tool (located just beneath the Pointer tool). Click on the underlying object first to set the focus, then click and drag as usual.