Check under Find Options, make sure XL is looking at values, not formulas?
Or, see if "exact match" is enabled. This would cause a search for "1234" to
not find the cell "1234 ".
Just because you can see it doesn't mean that it's there.
If a cell contains 123.999999 it may well display as 124. Cntrl-F and look
for 124 won't find it; Cntrl-F and look for 123 will find it.
It's worth clicking in the cell and seeing what the format bar shows.