Seatpost quick release question:
 

I’m having trouble getting my seat post tightened enough. When I tighten the locking bolt to where it should be it is still prone to side to side play. Not enough to move up and down, just side to side.

Is there something I am missing? Any suggestions?
https://cimg0.ibsrv.net/gimg/bikefor...32550f91e.jpeg
https://cimg1.ibsrv.net/gimg/bikefor...86c2a8e86.jpeg
Thanks!!!

Are you sure it's the seatpost -frame that has play.... or is it the top press fit part that your saddle mounts to?
Use carbon fiber assembly paste on the seatpost after cleaning it and the frame really well.

ETA: Is the seatpost too small for the frame?

Yep. The seatpost may be the incorrect size.

That would be my first thought too.

The binder bolt ears are really pinched together and may be touching at some point, preventing further tightening. The ears may be distorted but the cinch slot itself looks narrow, at least near the top. That typically indicates an undersized post. Unfortunately, I can't see the bottom of the cinch slot but I'd guess it's much wider at the bottom. which again would symptomatic of an undersize post.

I wonder if that friction compound they use on modern CF bike seatposts might help stop the slippage?.....

Your bike is a Trek? Pls confirm.

Incorrect seat post is probably the reason as many have said.

27.2 is the usual size.

I had this issue in the past and fixed it by drilling a hole (side to side) thru the seat post and the press fit saddle mount. Ran a tap thru the hole then secured the loose saddle mount with two short machine screws. Not terribly “unelegant” and definitely fixed the issue.

My experience with this exact type of setup has been that the seatpost has a particularly slippery anodized surface and that this rather stiff lug/clamp requires more than the usual amount of QR compression force to effect a normal amount of gripping force.

I recommend first applying something like WD40 to break down any old, slippery grease residue layer, and to also lubricate the QR lever cam with some oil.

Adding some fine grit from my bench grinder table to a thin layer of fresh grease would be the last step I would expect to take, with problem hopefully gone forever.

If the post head is actually slipping in the post I would definitely toss it. I've seen too many such slip-fit-glued or press-fit posts from this era develop cracks at the top of the post, which could suddenly cause a dangerous situation.

Hope you get your sizing issue solved if that's the problem. Once I figured out where I want the seat on my Mt. bike, I replaced the QR with a nice black hex key bolt. I don't want anyone else to quickly release it but me and am OK with the minor inconvenience.

^^^^^^^ Looking at how the ears are pinched, my money's on an undersize post.