Probably there is a syntax thing... a number containing a character, accidental spaces or non visiual characters.
Since you can't find it in the preview it might not be indexed correctly, can you find the product at all on any other property?
If you really want to be sure if it's actually in the index you can have a look at application called "Luke"
it allows you to check what is actually in lucene index files itself! Note you need to install Java, and there is not a lot of documentation. The version above should still work, but you can also check a newer take of same product: https://github.com/DmitryKey/luke