I also have so much trouble with sketches, when doing them on my iPad (which I use daily and which is up to scratch with all updates).
So what happens is: I make a sketch and save it. All looks good when I open it again, but when I continue to work on the sketch in 50% of the cases the updates to the sketch are lost when I open the sketch again at a later point, even though I absolutely did hit save (probably several times). Sometimes the thumbnail shows me the actual updated version but when I open it the updates are gone. Sometimes even the thumbnail doesn't show the updates. Last time it happend and my updates weren't saved, the version that Evernote had in the note was a copy I had saved directly on my iPad five days before and the work of five days was just gone. I tried to get it back -following the instructions I found online- but nope, all gone.
I tried to recreate the error, but I haven't found a systematic to it yet. It's just chance which updates to a sketch are saved and which not.
It's majorly frustrating and I decided to stop doing any sketches or take any handwritten notes in Evernote. It's too risky and too frustrating.