SQL - Problema con un query - UNION

Hola a todos,
Tengo un problema con un query que estoy realizando, se trata de dos selects unidos x un UNION cuando corro el primer select me trae 21 registros pero cuando lo corro con el otro select me trae 6, lo cual es muy raro pues la informacion principal la toma de tablas distintas. Alguien puede saber a que se puede deber o que estoy haciendo mal.. adjunto mi query... gracias

--unido a este select me lo reduce a 6 registros.
SELECT T5.PrjCode as 'CodigoProyecto', T5.PrjName as 'NombreProyecto',
T6.ItemCode as 'CodigoArticulo', T4.Dscription as 'NombreArticulo',
T4.Quantity as 'Cantidad', T8.AvgPrice as 'CostoPromedio',
T4.WhsCode as 'CodigoAlmacen', T7.WhsName as 'NombreAlmacen',
0.00 as 'Numero1',
0.00 as 'Numero2',
' ' as 'Caracter1',
' ' as 'Caracter2'
FROM OJDT T0, JDT1 T1, OACT T2, OMRV T3, MRV1 T4, OPRJ T5, OITM T6, OWHS T7, OITW T8
WHERE T0.TransId = T1.TransId
AND T1.Account = T2.AcctCode
AND T0.TransId = T3.TransId
AND T3.DocEntry = T4.DocEntry
AND T1.Project = T5.PrjCode
AND T4.ItemCode = T6.ItemCode
AND T4.WhsCode = T7.WhsCode
AND T7.WhsCode = T8.WhsCode
AND T6.ItemCode = T8.ItemCode
AND T2.FormatCode like '69%'
AND T1.Project = '%'
AND T7.WhsCode = '%'
AND T3.DocDate >= '01/01/2008'
AND T3.DocDate <= '01/31/2008'
AND T6.ItemCode = '0000158'

UNION
--este me trae los 21 registros
SELECT T2.PrjCode as 'CodigoProyecto', T2.PrjName as 'NombreProyecto',
T1.ItemCode as 'CodigoArticulo', T3.ItemName as 'NombreArticulo',
T1.Quantity as 'Cantidad', T5.AvgPrice as 'CostoPromedio',
T1.WhsCode as 'CodigoAlmacen', T4.WhsName as 'NombreAlmacen',
0.00 as 'Numero1',
0.00 as 'Numero2',
' ' as 'Caracter1',
' ' as 'Caracter2'
FROM OIGE T0, IGE1 T1, OPRJ T2, OITM T3, OWHS T4, OITW T5
WHERE T0.DocEntry = T1.DocEntry
AND T1.ItemCode = T3.ItemCode
AND T1.Project = T2.PrjCode
AND T3.ItemCode = T5.ItemCode
AND T1.WhsCode = T4.WhsCode
AND T4.WhsCode = T5.WhsCode
AND T1.Project like '%'
AND T0.DocDate >= '01/01/2008'
AND T0.DocDate <= '01/31/2008'
AND T4.WhsCode like '%'
AND T1.ItemCode = '0000158'

Intenta con Union All en lugar de solo Union.

Saludos.

Ponle UNION ALL

espero te sirva...