Guessing is never required for the puzzles, because it would defeat the purpose. Sometimes, on the harder puzzles, it may seem that more clues are needed, but generally on those you have to combine clues. For example sometimes it says "Alan had a day off later in the week than" and sometimes it says "Alan had the day after John off." When John is not off on Wednesday, then Alan can't be off on Thursday.
Sometimes you can combine several clues, for example if Alan is off earlier than John, and John is off earlier than the person who collects butterflies, then you know that Alan does not collect butterflies, and that Alan, John, and the butterfly collector must be in that order (not necessarily consecutive).
If you come across one you can't solve, take a screen shot and email it to me at email@example.com
, and I can walk you through it.