According to my 'Women's Home Companion Gardening Book' (1947) - you need to raise seeds. (The more recent recommendations agree because they say that the bulbs would rather get bigger than produce offsets. Haven't unearthed mine for about four years now so I can't say for sure.)

As well as - do you add compost to the area every year or so? They like a well-fed and free-draining soil. See what adding a pail of compost around them does.

Sometimes it can take several months for the benefits to show. Added now you may be able to improve water retention over the summer - which could be useful if your area has experienced a series of dry summers.

Vetivert's information squares with what I have read too. It is my understanding that the bulbs may get larger, but do not offset.

They like constant moisture, as said above.