Just wondering if this is a string length issue or character type issue - when you say numbers, do you mean short numbers such a 5 or 13 or 323 - or do larger numbers not work too, such as 20000 or 75000?
My thinking is that there is a general string length restriction (usually 3/4 chars [unless you alter it in the settings]) so if you're searching for smaller values such as 15 it may not work anyway because it is only two characters long.
It's the tokenizer you have it set to use (or just the default one). Most don't support indexing numbers (they will index alpha-numeric words though as you noticed).
This is the settings I use for digitalpoint.com which breaks apart words on punctuation (for example you could find a post that had "www.digitalpoint.com" in it by searching for "digitalpoint"... normally that's not the case), AND it indexes numbers with the pattern tokenizer.