Try squirting a couple of dollops of lube oil down the barrel. If you can make it roll through compression a couple of times with the starting handle off, then it's low on compression. It should almost be like hitting a brick wall (well, maybe not THAT severe....); I can spin my 6-1 up but if I let go the handle & knock the decompression off, it'll only roll through one compression, & bounces off the next. Also, if you can, keep cranking away at the handle* after you've disengaged the decompression. The more compression strokes you can get through, the more heat in the cylinder, the more likely you are to get it to fire.
*PS: Only do this if you've got a proper pawl mechanism on the handle, so if the engine spins faster than the handle, the crank rotates inside the handle. If they're locked together, you'll be in big sh*t if the handle's still on when it fires...