I have Young Cauliflower plants where something is eating or sucking the Sap out of stems of the plants but not the leaves. This causes the leaf to fall down.Plants are in a Tunnel and more outside Tunnel both infected.
Depending on the damage, this is caused either by slugs or a soil pest such as cutworms. You can look for the latter in the soil, just under the surface near the plants. If you can’t find these brown grubs, it is likely to be slugs or snails. If you find just a few cut worms, that can be enough and you just remove them.