Only getting botton half of page on report

I am creating s SSRS report from VB using a rdlc file.
When I display the report sometimes instead of getting the whole page I'm getting only the bottom half of the page displayed.
The top half of the page is off the top of the screen and cannot be accessed.
I have no idea why this is happening - any suggestions?
Thanks

Look for non-printable characters in your data, maybe there are LF or VT

It works OK on all computers appart from one.
So it must be an installed software issue - but I don't know which.

I have done some more work on this and found that the problem occurs on all computers.
I was just creating one or two reports and they were OK, other people were creating many reports and then they were not displaying correctly - you either get part or none of the report displayed - you xcan print it all.

Here is part of the code - I wonder if it is an initialistation or clearing problem that causes the problem.

ReportViewer1.Reset()
ReportViewer1.LocalReport.DisplayName = MyReport
ReportViewer1.LocalReport.ReportEmbeddedResource = "MIS." & MyReport & ".rdlc"
ReportViewer1.ProcessingMode = Microsoft.Reporting.WinForms.ProcessingMode.Local
ReportViewer1.LocalReport.ReportPath = AppPath & "\" & MyReport & ".rdlc"
'
'
' create a data adaptor
Dim da As SqlDataAdapter = New SqlDataAdapter(ReportQuery, SQLConnection)
' create a new data set
Dim ds As DataSet = New DataSet
' fill the data adapter
da.Fill(ds)
' clear the datasource
ReportViewer1.LocalReport.DataSources.Clear()
' set the dataource (DataSet1 is defined in the matrix in the rdlc file)
Dim rds1 As New Microsoft.Reporting.WinForms.ReportDataSource("DataSet1", ds.Tables(0))
' add a new datasource
ReportViewer1.LocalReport.DataSources.Add(rds1)

ReportViewer1.RefreshReport()