Sometimes spider mites will damage very young seedlings at that stage and stunt the growth. Spray the plant and the soil with horticultural oil. Other times it might just be the genetics of the seed.

Citrus seedling don't continually grow, they grow in spurts. First the top and than the roots.

I have about 4 or 5 that are the same way. Have been that way for two months, until recently. They are slowly starting to put out new growth.