Is the tape machine importing the songs in a format that iTunes can use, and a format that your iPod can play - AAC is the best for iTunes, followed by .mp3.
Your tape machine could be creating something like .wma files which don't play in iTunes (I believe). You may need a conversion program to convert the file to a compatible format (there are loads of free ones on the internet) iTunes is great, but the choice of file formats is, unfortuantely, a bit limited.