SQL Error - varchar to numeric

SQL Error - varchar to numeric

I imported a flat file into an SQL database and it created all of the
fields as varchar(50). I was able to change the data type of most fields,
but hit an error in the weight field. All of the data in the weight field
is less than 6 characters in total length and is either a whole integer or
a decimal. I have tried both:
UPDATE MAWB
SET Weight = CAST(Weight AS decimal(12,2))and:
ALTER TABLE MAWB
ALTER COLUMN [Weight] decimal(10,2)I always get the error: Error
converting data type varchar to numeric.
I've checked that all of the fields are considered numeric using:
SELECT COUNT(*)
FROM MAWB
WHERE ISNUMERIC(Weight) = 0I've also tried LTRIM and RTRIM just to be
safe, but still get the error when I try to change the field. Does anyone
know what else might be causing this error?