Plant stopped growing.

It's just trying to recover from the shock of the transplant... unless it's an autoflower, there's nothing to worry about. Good luck..

Most likely some transplant shock. Maybe something to do with any differences between the soil it was in and the soil it was added to, which will take a second to find equilibrium. May need a bit of N, since lower leaf is paling slightly... won't need much, but as the soil loses any nutes that came in it, you'll have to add more over time... Many variables are relevant so there's no "do this after 3 weeks" type precision in instructions... observe and react. Take notes for next time to pre-empt any obviuos trends related to the products and soil you use. That *could* be part of slow down, but it doesn't look extreme enough to cause something so noticeable. Still leaning transplant shock. Use more perlite or similar to improve drainage. Don't rough up plug before transplant. Any transplant shock should be a waring sign that something isn't quite right in the process, if it is more than an isolated occurence.