If you are sowing them in the garden rather than indoors you may be losing them to mice who just love them. They will eat the seed before they have time to sprout. I cannot grow any peas, beans, sweet peas etc., in the garden because of the abundance of field mice, voles and shrews.
The sweet pea seeds will also rot off quite quickly if they are kept too damp when sown. Good luck.
Mine are up to 4" now,when can I put them out in the garden?