Usually the reason is that devel-mode can only be enabled when you have set date to corrent one. Usually after reflash you have old date from factory image and most of the people forget to set the date/time before enabling devel mode. And if you enable 3G you usually get date/time automaticly updated from the operator and devel-mode install starts to work

Longer explanation: devel-mode package signature is 6 months newer than default date on factory image, and if you try to install it without setting the right date after reflash, you run into dpkg/aegis signature error, and UI just says not found