Practical pedagogy is called didactics and primary school teachers (should) learn a lot of it: after all, a child's brain is still quite undeveloped and you cannot teach them like you would teach a peer or yourself. E.g. you cannot teach grammar rules but you can teach a foreign language through singing, learning games etc. https://en.wikipedia.org/wiki/Didactic_method

These approaches work for most children most of the time, but when they don't, you have special education teachers who have a different degree in diagnosing (debugging) learning difficulties big and small as well as implementing interventions etc. The service they provide is also called remedial education [and it's especially cool when a primary school teacher and a special education teacher work in a big classroom together, the latter immediately bringing back to speed anyone who grasped something slower than the others]: https://en.wikipedia.org/wiki/Remedial_education