You mentioned switching USB cables so I’m assuming you are using Digirig Mobile.
The PTT options for this model is via serial CAT command (wouldn’t expect that supported in many mobile apps), PTT by RTS (more likely, but still not widely supported in mobile apps).
As far as PTT by RTS is involved in APRSdroid, please check out this discussion:
Digirig Lite is a better choice for the mobile apps with the range of PTT options optimized for mobile apps.
I found that. It sounds like that is the problem. I also saw that aprsdroid is working on adding support for RTS on mobile devices. aprsdroid PTT by RTS