Most software testing, in my experience, is rushed and often under time pressure, typically driven by a desire to meet a metric or measure of questionable value. Yet to rush through the testing of a feature or product is to often miss the changing nature of that product (or your own changing nature).
If you are testing a capability of a product and look at it in one way (say with the mindset of trying to break it) you may find several issues. If you shine a light on the product in another way (say with the mindset of trying to use the product really quickly) you may spot other issues.
Also, if you test the product on one day you may miss something you may spot on another day. You change (mood, energy, focus) all the time, so expect this to affect your testing.
The product likely hasn’t changed, but the way you see it now has.
Tours, personas and a variety of other test ideas give you a way of re-shining your light. Use these ideas to see the product in different ways, but never forget that it’s often time that is against you. And time is one of the hardest commodities to argue for during your testing phase.
How much time you will need to re-shine your light in different ways will mostly be unknown, but try to avoid being so laser focused on completing a test for coverage sake, or reaching the end goal of X test run per day, that you miss the serendipity that sometimes comes from simply stopping and observing and re-focusing your attention.