Keep trying! It took many days of trying to get my cat's ear to bleed, but now it does so easily. The ears really do "learn to bleed". How are you poking? Freehand or with a lancing device? I use Alphatrak2, and I have to have it on the highest setting (5) to get it to poke far enough (I trialed on my own finger, and on the lower settings the lancet didn't even touch it no matter how firmly I pressed it).
Paw pad is another option if your cat will let you go near his feet.