I am not sure if this will speak to your issue specifically, but the simplest solution is often the correct one, so here is what we have found.
The most common issue is that the device is locked to Sprint. Even if you put in a sprint card, the phone will often revert back to the activation screen because Sprint has not activated the phone on their end.
From my experience this is the case only when you leave a sim card in the phone once it is past the activation menu, if you don’t have a sim card in the phone, it should remain there without reverting back to the activation menu. If it still reverts back you may have a more serious issue.