What version of FW is on your watch and what SDK are you using to build the .prg?
If you build with the 1.2.x SDK, it won't run as a sideload on a watch that has the new beta FW with a 1.3.0 or 2.1.0 CIQ VM. You'll see a "signing error" in ciq_log.txt.
There's a sticky post at the top of this forum that talks about app signing.