you have a fixed gear bike, no give at all in the drivetrain. backspin the wheel against pedal resistance, wheel in the air (on a stand if you have one, broom handle between two chairs if not. if you feel slippage you know there's a problem, can start to look for the causes


cog too narrow
you can visually check the cog width looking straight down from above

the hub has two threaded sections, one for the cog, which threads one way and is wider in diameter (from the axle). the lockring threads are smaller in diameter. if you've tightened the lockring all the way and there's still a gap between it and the cog then the cog is too narrow, which would allow it to loosen with backpressure. that's a visual check you can do.

lockring incorrectly tightened
to check the other option you have to take the wheel out and use a chainwhip, or rotafix it in situ, to make sure the cog's threaded all the ay on, then tighten the lockring to it.

does that make sense?