Channel balance replacement - Peachtree

I am changing the channel balances of the lower sash in a Peachtree double hung non-tilt cottage style window. Markings indicate 3672 1G, no stamp. I weighted the bottom sash at 24lbs. Old channel balance is 32.25 length, width 1/2in. I bought a new channel balance to fit D3672 window size, weight capacity 24-28lbs with 31.25-4 stamp. Once replaced, the window will not stay up, it falls. The weight of the window isn't being held up but for maybe 1/4in. from the bottom and I have to push down to close.
I replaced channel balances in a different size window that was not cottage style. Everything works fine in that replacement. Is it possible I need to go up in weight capacity for the cottage style window?
Thank you.