Channel Balance Install Trouble

Hi, I ordered 2 channel balances (Item # S385-17, option 1630). They are almost identical to the broken ones in the window, numbers are correct. So, install was easy, but as soon as I close the window completely, the hooks pop out of the slots in the window frame with a twang. I checked to be sure I have the hooks in the proper slots; they are. 3 tries with the same result. Can you help me figure out what I am doing wrong? The channel balances seem a little loose in the window channel. Is it possible that I have the wrong part? It sure looks right. I believe that the windows are Milgard, non tilt.
Thanks

Hi Tom, Thanks so much for your quick reply. Here are pictures of the old channel balance (on the right) and the new parts from SWISCO. The differences seem to be the length of the plastic tabs at the bottom and a difference in the top of the balance that nestles under the clip in the window frame. Again, install is easy until I press the clips back in and move the window up and down. Then, with a twang, the clips come out of the holes that they were hooked into. Thanks again for your help!