Game Won't run through Steam

We have had a number of alpha testers (about 20%) unable to play our game. When they launch it through Steam, it runs as a background process and never opens. If they try again Steam says already running. We are unsure what to do.

We have never had this issue outside of Steam, and the users who are affected are able to run the game if they drag it to another folder and open it outside of Steam (but then they can’t play online). We aren’t doing anything fancy with Steam, just host game and join game. Would appreciate any help!

Have you requested a client log from these users? It may help you find out what is wrong.

We plan to do that next. We discovered that the problem seems to happen to anyone who doesn’t store the game in their C drive. Any ideas what could possibly cause this?