What plug-in are you using?
That's not a plugin. Would you explain further how you are importing?
Could you also describe what happens in a lot more detail, including the part where you say "a lot of my images are only thumbnails"? I don't understand what you mean. All images in Lightroom show thumbnails in the Library Grid, and should have the full sized image available for editing and exporting.
From the file menu I select Plug in extras and then select Import from Aperture Library.
When I select a thumbnail from the Library grid the image I select is a very small image that can't be zoomed in or out of and appears to be un editable.
It would help to know what you're looking at.
The Aperture importer gives you the option to import previews; did you do that? If so, it's possible that what you are looking at is a preview of a master, not the master (those are what Aperture calls them).
A screen shot would be helpful. Do you have a Lightroom Masters folder in the Finder? what happens when you view a folder within that in Lr (NOT a collection, a folder)? What module are you working in?