letters filling in when converting to AI file.

We receive art files here to work on. They've gone to a couple of different sources before we get them, so we really don't know how the file was originally saved. It gets emailed to us so that we can get it ready to send to the plate makers.

After opening the file, I save it as a Corel Draw file, then work on it. The problem starts when converting the Corel file to an AI file. The AI file now shows sections of letters filled in, like the letter o. 

Does anyone have any ideas?

We're using Corel X5 and using Windows 7.

Thank you so much,