The outer covering of the sweet pea seed is hard and resistant to absorbing water, an important precursor to seed germination. There are several methods to getting around this. In the early spring place the seeds on a damp paper towel in a warm spot for several hours. Wait for the seeds to swell before planting promptly. Any that have not swelled up can be nicked gently with a nail file or knife and placed back on the paper towel until they too begin to germinate.
Alternatively nick the outer coat of each seed before planting or place in a jar with an ample amount of sand and water, shake vigorously to scratch the seed surface. This will allow water to enter the seed readily.
The easy way and far more rewarding is to plant your sweet pea seeds out in the garden in the autumn. This mimics the lifecycle of the wild sweet pea plant. Our wet autumns are ideal for soaking the seed in the soil where they will begin to grow slowly over the winter. In the spring be prepared for a burst of green and an early bloom just when other gardeners are planting their seeds out after laboriously preparing the seeds.
If you leave some of the blossoms to go to seed sweet peas will reseed themselves just as their wild cousins do, but be sure to plant out some new seeds too to keep up the show of flowers and scent.