I'm in a very similar situation. My partners have other business besides the startup and I believe this is one of the main problems that's blocking our growth.

I suggest you: (1) to define tasks for your partners on a board (like trello), then just follow the tasks if they are being executed. (2) to have a limit date for the startup to profit and grow, and if the date is not met think about leave the startup.

Time is the most valuable asset. Use it wisely :)