Why does our team keep mishandling and losing track of samples?

You lose track of a sample. You mislabel one. It happens again next week, with a different person at fault. You tell yourselves to be more careful, and nothing changes, because carefulness was never the problem. The problem is that each of you handles samples a little differently. One person labels first, another logs first. No one checks another's work until something is already wrong. A team without one shared method will always produce errors — not because anyone is careless, but because there is no single way to do the thing. You cannot fix this by trying harder alone. You fix it by choosing, together, one exact process — and by making someone other than the handler check it, every single time.