Strange issues booting layer1

Hi all I have compiled osmocombb and everything seems ok. Now i'm having a strange issue which I can replicate across a number of phones. Using a C118 I could boot everything and run the mobile app. Using a C139 I could do the same. However, now the phones dont seem to want to boot? I start everything up (after running the mobile app a few times, stopping the phone and PC etc) and then when I press the Power Off key, nothing happens. It just refuses to download anything. Am wondering if i'm damaging the phones somehow as its now two phones that will no longer start via the USB cable. The phones boot the normal phone firmware fine but wont respond to osmocom. Guess i'm just curious as to others thoughts as i'm unsure where its breaking?