INSERT INTO SELECT JOIN THING

Hi there!

I'm working in MSSQL Server 2008 R2

I have a table "AllQuarters" with a list of distinct employee numbers [EMPLOYEE] and fields for quarterly wages for those employees like so:
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EMPLOYEE | Q1_GROSS_WAGES | Q2_GROSS_WAGES | Q3_GROSS_WAGES | Q4_GROSS_WAGES |
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I also have 4 tables (Q1 - Q4) that have the employee number [EMPLOYEE] and the gross wages [GROSS_WAGES] for the employee for that quarter.

I'm trying to join the Q1 - Q4 tables to the AllQuarters table on [EMPLOYEE] and insert the [GROSS_WAGES] from the appropriate quarter into its respective [QX_GROSS_WAGES] field in AllQuarters.

Not all employees exist in all Q1 - Q4 tables.  

I've tried INSERT INTO SELECT with a JOIN but my results end up with a bunch of NULL values.  Not sure what I'm doing wrong.  Any help will be greatly appreciated.  

TIA!