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It's probably not a surprise that newer, faster phones work better. This is because they usually have better cameras, faster processing chips and use the newest network connections. While a new phone drastically improves performance, the main issue is the location of where you are scanning. If you have poor cellular connection at your scanning location the app might not work. This is one reason we encourage you to test the exact location prior to any scanning days. What you might discover is that one carrier, Verizon for example has a great connection at that location but those using AT&T have a very weak and slow connection making the scanning impossible with that carrier at that location.
When you experience problems, note which phone hardware, the version of the phone software and who the carrier is. You will quickly discover if there is a problem with one of the carriers and/or models of phones. One of the biggest problems I've seen is phones that are not on a major cellular network. Make a list of these poor performing phones and carriers. When recruiting volunteers ask about their phone and carrier to head off any location specific issues you've discovered.
Now let's assume they are on a proven phone and network but it's still not working correctly. Try these one by one until your issue is resolved.
If you are still having trouble email me a detailed description of the issue along with the following info. App Version.