Magic Wand problem in PSE 8 - unexplained gaps in selection

Question

Hello,
it seems that recently when I select an area of a shot using Magic Wand (MW) and create a Layer with the selection, there are unaccountable partially missing "see through" bits where I can see some of the check fill pattern through the selected area.
I have been using PSE 8 for a few months and I have never noticed this before but perhaps I was not looking hard enough. MW also seems to be using tolerance settings a bit differently but that is not easy to demonstrate - it just seems that with a given tolerance it takes a lot more Shift-clicks to eliminate spots inside the desired area and still some remain, though feathering seems to clear them away. I often select an area by building it up with a lot of Shift-clicks and usually decreasing tolerance values as I go, also using Alt-clicks to remove unwanted inclusion. Until now once all the "ants" were eliminated from inside the desired area I could then feather it and with Ctrl-C and Ctrl-V achieve a layer with no discernible holes. (I usually feather with Refine Edge rather than the Feather option because that way there is no fade off where the edge of the shot is selected.)
Does anyone have any idea what could be causing these gaps?
Thank you,
Nick Sergeant

Answer 1

Thanks, Terri_F and hatstead,
following your suggestions I have just experimented with all six combinations of Eyedropper (ED) and Magic Wand (MW)
ED = Point Sample, MW with/without contiguous checked
ED = 3 by 3 Average, MW with/without contiguous checked
ED = 5 by 5 Average, MW with/without contiguous checked
These setting do make a difference but the MW select is always leaving out at least a few stray pixels. But the problem is not a show-stopper as I may have a work-around (below)
I think ED= 5 by 5 with MW contiguous was the most reliable for picking up almost all of a clearly isolated area of colour. Even that setting leaves some stray pixels out for some reason but nothing serious. All other settings seem to leave holes in the area selected despite no sign of them during the selecting. 
I ran some settings more than once and despite this systematic approach it seems I can never get quite the same result twice. I'd expect the edge of the selected area to fluctuate a bit each time, but I also still get little few-pixels-sized holes well inside the area seIected, which spread out of course if I feather.
I think what I am seeing looks worse than it really is because tiny groups of a few pixels each left out of the select will expand when feathered and be quite an eye-sore. So from now on in these cases, if necessary I may work-around by Select > Modify > Expand for one or two pixels in order to capture stray pixels, and then feather. I tried it once and it seemed to capture all the stray pixels without compromising the select outline too much.
Thanks for your help,
Nick