Until 6-7 years ago, I always experienced a certain "dropout" when I planted cabbage and lettuce as a second crop during the summer. The seedlings could get into the ground one day, and already the next morning one could find a single or two lying flat down, with the stem gnawed over right into the soil surface. The culprit was usually the budworm, the barn owl's fat gray larva, which is found in the ground during the day, but which climbs to the surface at night and likes to gnaw on small plants. If you dug a little into the ground around the fallen plant, you could occasionally find it - if you did not, you could be almost certain that it found a new victim the following night.
However, unlike snails, for example, the snail is not very firm to climb over an obstacle - when it bumps its forehead against a wall, it can do nothing but just unconsciously change direction. Therefore, a simple barrier in the form of a 3-4 cm high plastic ring has proven to be largely 100% effective. The rings are sawn out of, for example, a piece of plastic downpipe and pressed firmly into the ground around the plant - and if you use 75mm pipes, the ring can easily be left even around large cabbage plants. As an extra side benefit of the method, it is easier to water the seedlings in the beginning if you run into a drought period - you just have to aim with the beam so you hit within the ring.