Andersen window sagging

A quick learner from Missouri says:
I just replaced the balances in two of my Andersen windows, using your Andersen 24-737 and 24-1033.

The window with the 737s works fine.
The lower (front) sash using the 24-1033 works fine, but the upper sash on that window sags.

What could cause this?
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Tom from SWISCO responded:
It's possible the top sash does not use the 24-1033. The top sash and bottom sash do not always weigh the same. Can you confirm what numbers are printed on the original balance boxes that once supported the top sash? That will allow us to find a better match.
A quick learner says:
Good morning, hope you get this e -mail from the end user. This anderson window is not unusual. DH sash opening is 36" wide and 61 1/2" tall with equal sized balance unit appeared to be a replacement unit w a number of 1031 S37 07 06, and the other appeared to be an origional balance with a number of BFG 10#31 2 87, which still had a staple affixing it in place. Additionally the home was built in 1987 which I believe the 87 in the part #corresponds to. The other clue that I can add is that when I pluk the cords the upper sash cord sound a bit different than the lower sash cords, like a guitar string, lowers are not the same note and are higher in pitch.
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Paul from SWISCO responded:
You are correct, the numbers printed on the balance (1031) indicate that our 24-1033 is the right replacement, but the window size tells me you should have the 24-1133 instead. You see this from time to time when manufacturers or contractors install windows. They will be out of the correct balance so they substitute with another. It's almost impossible to know for sure when this happens, unfortunately, but it is uncommon. Did you ever find the windows to droop before with the old 1031 balance?
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