I occasionally have the same experience. I can't explain it but sometime I just can not get a good lather from the soap. Then the next time I use it, it works just fine. I just assume the shave gods are telling me to use a different product.
This actually goes for all of my soaps. I generally have no problem getting good lather from a cream, I tend to struggle every now and again with the soaps.
My name's Jim, but most people call me...Jim.