Hello.
I have the same problem in my raised beds after some months of unprecedented drought in West Wales.
I have 15 very deep raised beds (around 50 cm deep) sited on an area which was excavated leaving only sub soil. The beds were filled with bought ‘topsoil’ with little humus development and pockets of clay. The beds were then cultivated conventionally for around 8-9 years, using compost and occasional dressings of manure.
Over the last 2-3 years I have followed the no dig method. Each year I’ve dressed the beds with 2-4″ of home made compost, or farmyard manure or a compost made from local food recycling. But this year its hard to get a dibber to penetrate and the hardness of the soil seems to be affecting the root development of some young cabbage and lettuce plants. They grow on well at the start with but then fall over. When harvesting lettuce I noticed their roots are undeveloped, not wide and branching, so I’m concluding that the roots are not getting a purchase in the tough soil. I’m watering most nights but not too much as we are on private water which is getting low so we need to preserve where we can. I also have cracks at the edge and in the middle.