I have had the same problem with some of my seeds.
Once it comes up and I can see that the cotyledons cannot emerge from the hard seed coat, I just take a pair of pliers and crack the seed. The smaller the seed, the smaller the pliers. I have even done it with tomatoes that can't drop the outer husk of the seed.
All you want to do is crack it hard enough to cause the outer shell to split slightly. The seedling can usually do the rest of the work.
Cheers,
Carolyn