Skoda Kodiaq 7 seat seat belts

Since having a 3rd child, my 5 and a half year old son (19.5kg) is sitting in the boot seat. He is in a seat belted high back booster seat. We have had two occasions of sudden breaking and his seat belt doesn't seem to engage and he has banged his head on the seat in front of him. Any suggestions as to what we might be doing wrong? Things we need to change please? When we test the seat belt by giving it a tug it locks in place as expected.