How can i populate colonies?

I am currently at 1490s, playing as England and i have colonised all the Caribbean under direct rule (no colony subjects) and some of Canada i use for a base for further exploration. My plan is to fully upgrade all the RGOs in the caribbean and use my trade capacity in London to import goods and then export them to Europe.

I tried populating the Carabbean by importing slaves from Morocco to London and from London to the Carabbean market i created, however, all the slaves that go there are just being exported by local burghers (although all the locations in the market being rural) to other native American markets while the autonomous colony in Canada has demand for slaves.

Should i increase the trade of slaves even more or is it some country law that doesn't let me use slaves?