From my experience practising IELTS reading with students, skimming and scanning are techniques that don't usually help. When students try to skim or scan, they often miss the answers completely.
For example, if you are scanning for the word "buy" but the passage contains the synonym "purchase", you probably won't find the answer.
So what is the solution? Instead of skimming or scanning, I tell my students to read at normal speed. Only scan quickly if you are searching for a name or a number.