If this happens to you, it's likely that your phone’s GPS accuracy was not good enough at the time for it to pinpoint you at the stash.
If you open Apple Maps (or Google Maps) and zoom in, you can see a semi transparent circle around you. If that is big, or your blue dot moves around a bit, then it’s a guess that you are somewhere there. If that accuracy is just not good enough for us, it will not trigger the stash to open.
If this happens, try this: Open Apple Maps, leave open for a minute, then flick back into Aglet. It may help reset your GPS to have a higher degree of accuracy.