Query design causing locked table / odbc errors

I have a view called view_quotation_list which is pretty much as it says, a detailed list of quotation information. Now these 'quote' records can have a non-unique quote_number and I created a second view called view_quote_totals which totals the values of records with the same quote_number as shown below:

SELECT quote_number, SUM(CAST((ISNULL(door_qty, 1) * quotation_total) * (1 - CAST(ISNULL(discount, 0) AS DECIMAL) / 100) AS DECIMAL(9 , 0))) AS quote_value
FROM dbo.quotations
GROUP BY quote_number

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It seems that by simply linking the totals view to the list view (by the quote number field) causes some sort of locking with major problems for all database users. My Access front-end is unable to save changes to the quotations table and ends up giving a query time-out ODBC error or communication link failure error. I should also say that I am using a pass through query in Access to return x records from view_quotation_list as follows:

SET DATEFORMAT dmy;SELECT TOP 200 * FROM dbo.view_quotation_list WHERE latest_revision=1 ORDER BY quote_number DESC, item_number ASC, revision_number DESC;

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If I remove the totals view, everything works fine again so I tried putting the select statement shown below (which calculates the totals) into the original list view but the same 'locking' happens again.

(SELECT SUM(CAST((ISNULL(door_qty, 1) * quotation_total) * (1 - CAST(ISNULL(discount, 0) AS DECIMAL) / 100) AS DECIMAL(9, 0))) AS Expr1
FROM          dbo.quotations AS q
WHERE      (quote_number = dbo.quotations.quote_number) AND (latest_revision = 1)) AS quote_value

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It's as though there's a recursive process going on maybe due to my query design? Any thoughts on alternative solutions or insights into where I've gone wrong most welcome. Thanks