Macro Debugs When Run Via CommandButton

I have an unusual problem with my excel workbook. The workbook consists of 2 sheets, "WorksOrder" and "Data". In the WorksOrder sheet, the user enters some WorksOrder number and then hits a Commandbutton to retrieve the data from SQL and stores it in the "Data" sheet. It then goes and copies the values from the Data sheet. The problem occurs when it's trying to copy a cell value in Data into a WorksOrder cell. The error I get is Run-time error '1004': Application-defined or object-defined error

The weird thing is, when I'm stepping through the code, I don't get the error message and the code works. It only happens when the code is triggered from the Commandbutton. I've tracked the error to a variable which supposed to have a row number greater than zero but instead has a zero (0) value. My question is why isn't the function returning the value to the variable ThisRow? The code for copying the cell data is listed below:

Dim ThisRow As Long
Dim ThisWorksOrder As String
ThisWorksOrder = "123456"
ThisRow = FindRowNumber(ThisWorksOrder)
Worksheets("WorksOrder").Range("D1").Value = WorkSheets("Data").Range("H" & CStr(ThisRow).Value.............