I had the same problem and if your seat has any lateral movement be on the lookout for a cracked crossbar like below:
I actually had this happen on the original seat on my car too. I did nothing about it and eventually the entire seatback fell off. I guess the swaying caused the welding from the base to the back to crack. Imagine driving down the road and having your seat fall off
As you can see in the picture I put in a "L" shaped bracket to stop the sway.
Hope this helps,
Kevin