Time keeping match day.

Time keeping. I ask players to turn up 30-40 minutes before a game but with 20 minutes to kick off I’m still missing half a squad. I give plenty of notice to parents about how time keeping is so important as we need to warm up as a team and discuss the game ahead. This has happened On more than 1 occasion. We losing games quite regular now as feel the focus is not there. Any advice is welcome.