[DJ Dubovsky]

This is a tip mentioned by Doug Nelson. If if you resize your image the history brush will only work from the point were you resized forward (It cannot work on differently-sized image)
If you plan to use the history brush at some time in your digital manipulation, it would be a good idea to make a snapshot after you do any resizing of the image that also includes using the crop tool or rotate canvas.
DJ

[Doug Nelson]

I'll add for the uninitiated that 'snapshots' are static copies of your image that can be referred to by the history brush at any point, even if you've gone past the maximum number of history steps you've allocated (or used to revert the entire image). You can have as many snapshots as you like, but be cautious as they'll bloat your file very quickly.

[DJ Dubovsky]

Don't forget to name those snapshots also for easy reference.
DJ