they seem to be growing very slowly
I would be patient and wait at least another month.
Generally this years 'growth' is around three to four weeks behind what it would have been in the last few years.
These are seedlings I sowed 21st January and pricked out on 20th February then they just more or less stopped growing ( like lots of my other stuff). I would say that they have put on more growth in the last couple of weeks than they did the previous 10 weeks.
Over the last few years my plants would be at the stage where I was potting them up into 5" pots now.
So as I said........" patience" !