Windows form not always recognized in property checkpoint

For debugging this problem I would be tempted to try using FindChild in a loop to make sure TestComplete is really finding your form.  Once it's found then you could use the property checkpoint to make sure it's visible on screen.  

I would use a timer in the loop so that you aren't stuck forever if something goes wrong.  You could use something like this to wait for the form:

Function WaitForChild(parentObj, childName, timeoutMS)
  Dim startTime, elapsedTime, childObj

  startTime = aqDateTime.Now()
  Do
    Set childObj = parentObj.FindChild("Name", childName, 5)
    If Not childObj Is Nothing And childObj.Exists Then
      Set WaitForChild = childObj
      Exit Function
    End If

    Delay 100 ' Wait briefly before checking again
    elapsedTime = aqDateTime.TimeSpan(aqDateTime.Now(), startTime) * 1000
  Loop While elapsedTime < timeoutMS

  Log.Warning "Child object '" & childName & "' did not appear within " & timeoutMS & " ms."
  Set WaitForChild = Nothing
End Function