A spongy brake lever, or a brake lever which has to be pulled a long way before you feel the brake start to work, is a sure sign of air trapped in the brake system.
Some brakes can be more troublesome to bleed than others. Even after multiple bleeds air can remain trapped inside the caliper. Perseverance is the key to an air-free brake.
Thanks for that, but you're not helping.
OK. Think about the orientation of your brake lever and caliper. Some bike frame designs can see the caliper mounted in weird and wonderful positions.
In the picture below this Shimano brake caliper is mounted between the chainstay and the seatstay resulting in the bleed nipple (circled in red) to be at the lowest point on the caliper. The ideal spot for the nipple to be in during brake bleeding is marked by the green 'X', at the highest point on the caliper.
This brake caliper needs to be temporarily removed from the frame so that it can be rotated to the proper position during bleeding.
As air inside a fluid will always rise to the top, make sure that any exit points (bleed ports or bleed nipples) are at the highest point in the brake system. This will help to remove any trapped air bubbles and encourage them to exit the system.
It also helps to gently tap the caliper or hydraulic hoses with a screwdriver handle or similar to help dislodge any trapped air bubbles.
If all else fails keep trying. You'll get there eventually. :)