Trimming dog's nails problem?

I have two dogs, well I own one and my sister owns the other. My dog is a poodle and it is easy to clip his nails. The Jack Russell is a pain as they are shaped differently and they are black.

Today I went to trim back his nails and it sounded like the first nail cracked up the nail. I am using these trimmers and I had just clipped the poodle’s nails with no problem.

Now I am very concerned about continuing for fear that the crack will continue up the nail and cause the poor pup pain.

Does anyone have any ideas on what I can do to stop that from happening?