Channel balance issue

My window sash is about 27 X 34. The old channel balance is 2640. I've ordered replacements from you but they don't work so I'm wondering if the channel balances we replaced were perhaps not the original. The new channel balances do not disengage from the take out clips when we pull down the sash. They're just not connecting to the window. Looks like the window just goes right over the white shoe attachment on the channel balance without catching on the shoe. Should we try a wider channel balance, day 3/8 instead of 1/2 inch?
Could the spring weight be wrong? Could the shoe attachment be wrong?