I think what you are saying is that you've made a selection but some pixels are not selected in the middle. This is common with the Magic Wand, which selects pixels but color and leaves others behind.
One thing you can do is to paint those pixels in. Try this:
Select menu > Save Selection (this saves the selection as an alpha channel)
Open the Channels panel and select the alpha channel
Type "D" to ensure you are use pure black and white
Use the Brush tool "B" and paint the missing pixels into the alpha channel
When finished, select the RGB composite channel
Select menu > load selection > load the alpha channel
If you get stuck, ask again and show a screen shot so we can see what you are seeing.
Hwen yoy make a selection for instance with the magic wand there are some holes in the selection. Or when you make a color range selection, there are some tiny bits that is not in the selection.
So I would like a button or a command that mends the selection to be without holes.
Both Magic Wand and Color Range select by color, so if some pixels aren't the same color then they won't get selected. You can change the tolerance to select more pixels or you can Shift-click to select more. Or you can paint in the missing pixels in Quick Mask.