Strange GROUP BY Statement Output

I am currently straching my head as to why the following doesn't work as I expect.
I have Three tables UsersManagers, Users and Bookings:-

SELECT MAX(UsersManagers.UsernameUser) AS UserID, SUM(Bookings.HoursTotal) AS NumHours
FROM UsersManagers
LEFT OUTER JOIN Users ON Users.Username = UsersManagers.UsernameUser
LEFT OUTER JOIN Bookings ON Bookings.Username = Users.Username
WHERE UsersManagers.UsernameManager = 'testPM1'
AND (Bookings.DateOfBooking BETWEEN '01 October 2006' AND '31 October 2006')
GROUP BY UsersManagers.UsernameUser

This statement returns only one username with the hours they have booked in my Bookings table. However the left outer joins (I would have thought) should return an entry even if there are no hours booked for a user. If I omit the Date Section:

SELECT MAX(UsersManagers.UsernameUser) AS UserID, SUM(Bookings.HoursTotal) AS NumHours
FROM UsersManagers
LEFT OUTER JOIN Users ON Users.Username = UsersManagers.UsernameUser
LEFT OUTER JOIN Bookings ON Bookings.Username = Users.Username
WHERE UsersManagers.UsernameManager = 'testPM1'
GROUP BY UsersManagers.UsernameUser

I get two records returned
testUsr1 37.5
testUsr2 NULL

Why don't I get this when I use a date?

Thanks