The Find function has many properties. Most of them are set by Excel automatically, depending upon last use. Even if you last used Find manually, Excel will remember your last setting. Therefore you should set all relevant properties when using VBA.
Sub FindCell()
Dim foundRng As Range
Set foundRng = Range("F1:F2000").Find(What:="9999", _
After:=Cells(2000, "F"), _
LookIn:=vbValues, _
LookAt:=xlWhole, _
SearchOrder:=xlByRows, _
SearchDirection:=xlNext, _
MatchCase:=False)
If foundRng Is Nothing Then
MsgBox "Value not found"
Else
MsgBox foundRng.Address
End If
The likely source of your problem is the LookIn property which I suspect your system currently uses xlFormulas (it's xlValuesabove). When a cell has a hard value, like 9999, its formula and value are the same. But if the formla is =3*3333 value and formula differ and 9999 will not be found in the formula.
The other item worth mentioning is the After property. Excel starts the search in the cell after the specified cell (by default the first cell in the range). The next cell after is specified by the SearchDirection property (above xlNext, could be xlPrevious). The function will loop through the entire range. Therefore, by specifying the last cell as after and xlNext as direction, the search will actually start in the first cell whereas, if the first cell is specified as After it will, in fact, be the last cell to be examined.