mythril nuggets
You didn't get support did you? Old recipes still need them. Not having the proper manual or support adds a flat (does not vary with level) amount of "risk" to your "small & negligible" random starting value.

My guess on risk is somewhat like this...
Starting Risk = Base + RandomValue + Manual + Support
where Manual and Support are 0 if you have them and reduced if insufficient
Base gets smaller as your level goes higher.

In any case, materials are dirt cheap in XIV. Get over the occasional breaks.