seedling stayed same size for 2 weeks

the seedling germinated 3 weeks ago.. and grow abit then stopped. I have no idea would the problem is? roots?

It could be any number of things (a picture always helps) but common causes are low temperatures, improper pH, over ferting, insufficient light and bad air flow. Seedlings also like the humidity kind of high and not to mention some genetics begin each stage at a crawl before exploding with growth. A little more information about your setup should help.

what soil?

I have a clone near 40 days from root stunted cause the soil was too hot. Not burnt just puny and not growing.
Up pot with new soil mix and it has taken off.