If it’s not the detent spring then consider this. Rockets need to “clunk” into first gear. When you start the bike ALWAYS let the clutch out with the bike in neutral before engaging first. It should clunk. If you start in gear, pop it into neutral, let the clutch out and then go back into first. If when you engage first and don’t get that clunk there’s a good chance it will pop out of first gear, especially under load. Generally if you engage first while the bike’s still rolling, say to a stop, then it should engage properly without needing to let the clutch out in neutral. Don’t question it, just do it, every time.