..and you need to be very careful with watering. Peppers roots will rot off easily and other thing they don't like is getting their roots hot in a plastic pot. You can see them avoiding warmth by having fewer roots on sunny side of the pots so re-potting into slightly larger container will help this and their roots will soon romp away.
It could be also down to nutrients...if they've been some time in same compost..it could be that there is not much nutrients left there to support new root development.
Or it could be just 'one of those things'....everything in good time!
Just like craggle said....re-pot..