If I remember one of the major issues with home brewing one's own fuel was the quality of the ingredients they could get.
Pump diesel for example can vary a lot from shipment to shipment. Kerosene has the same problem with big variances from batch to batch.
Our gasoline changes over twice a year in order to have a winter blend and a summer blend, and the differences can affect your engines.
So you might have a batch of DIY fuel that works now, but when you run out and go to get more ingredients, your next batch may not work well at all.
It isn't your formula that was bad, it is the ingredients changed from when you bought it last.