Side pin ribbons won't clip onto motherboard

I've had some old joycon lying around that I decided to fix recently. I bought a replacement for the SL/SR side rail board (the one with the sync button and player LEDs), and I clipped it onto the board like normal, and doing the same for the handheld connector pins' ribbon. Resulting, the joycon could barely connect to the switch when testing. The connection is intermittent and rarely works at all. After some inspection it turns out that the ribbons, even if clipped down to the board, will still slide out easily with a bit of fenagling, which may have something to do with the spotty connection. Are these ribbons supposed to be as loose as they are?

1. It's not the battery. I've swapped this joycon's battery into a joycon I know works, and both batteries charge OK.
2. The replacement rail board I ordered is working. Again, I swapped it into a joycon I know works, and the replacement worked fine there.
3. I ordered the correct left/right variant of the board.

HAC-JCR-MAIN-01 [R]007-AE0046