After the first level, the game is made more difficult by having the ground targets shoot back at enemy planes. But the logic only targets the first plane on the level (player 1, the cyan player). This should instead be searching through the object list to find any enemy planes that are in range.
See here: https://github.com/fragglet/sdl-sopwith/blob/ea5b00ae3d16f990cc42b0a5e971dd3b4d2f96d3/src/swmove.c#L859
After the first level, the game is made more difficult by having the ground targets shoot back at enemy planes. But the logic only targets the first plane on the level (player 1, the cyan player). This should instead be searching through the object list to find any enemy planes that are in range.