Since nobody else has got back to you..

When the printer is unhappy with the quality, I suspect they actually mean the resolution of the image. They expect 300DPI, so for a ten-inch t-shirt design the width would be 3000 pixels. Check the size of the graphic image that was sent.

The export to AI is probably flattened as a bitmap so it will be uneditable. It's like loading a jpg into a Xara file - it will be a single thing.

Check the export options for ai - particularly the DPI and output document size.

Hope that helps.

Paul